The Stovebolt Site Forums The Shop Area Engine Shop Slight tapping noise at higher RPMs on a 327

Hey everyone...

Still working on my 1966 C30. She has the original 327 in her with 27,000 miles.

I have noticed a slight tapping noise at higher RPMs. The noise is not there at low RPMs. Honestly if I had a loud exhaust, I bet I would not hear this.

I looked for any blow by at the exhaust manifolds but found none. She does not burn any fluids. I am using 20w50 in her per the previous owner's instructions.

Any idea what this noise might be? Am I harming the engine? Can I use one of those engine treatments for "Noisy Valves"?

Thanks again everyone... Russ

Should I be using the "Lead Substitute" in the gas tank?

There are quite a few possibilities for your noise. Some questions:

1. At approximately what RPM does the noise appear?

2. Does engine temperature make any difference?

3. Does throttle position make any difference? Light throttle; heavy throttle?

The possibilites are lifter/tappet noise, loose piston, loose connecting rod, detonation.

Detonation will often increase in intensity with a light increase in throttle, and may diminish with heavier throttle. If that happens, disconnect the vacuum advance and see what happens. If there is no more noise, the timing may be too far advanced, or the vacuum advance may be coming on too much. If the noise gets louder with heavier throttle, advanced timing might still be the cause, so check that and retard the initial setting some to see if anything changes.

A loose piston will usually show up as a light tapping sound at light throttle and at moderate engine speed. The sound also doesn't change change much with throttle position.

A connecting rod knock will usually show up at light throttle, at moderate engine speed, and often worsens as you lightly release the throttle. Rod noise usually disappears completely during deceleration. Depending on conditions, rod noise may also diminish or disappear completely with heavy acceleration.

Lifter/tappet/rocker arm noise is normally constant, regardless of throttle position, and sometimes doesn't appear until above a fast idle. But that situation isn't very common. Lifter noise is more likely to appear at low engine speed and to disappear as engine speed increases.

If the noise exists mostly with the engine cold, piston slap (loose piston) is a possibility. But lifter noise is also common with cold engines.

I don't know how much this helps, but it gives you something to look into.

Harvester

Fuel pump spring broken? I've seen a bad fuel pump make some strange noise.

Good point, I've seen that one also.

Harvester

Hey Harvester...

I don't think I notice the noise as much when the engine is cold. I only notice the noise on the highway when the rest of the truck is quiet (Loud transmission and other cab noises). By the time I can get him up to higher RPMs (Highway etc) the engine is basically warmed up. I have a car rally near a highway next week. I can see then if the noise happens with a 'cold' engine at highway speeds.

I don't know the RPS.. no gauge.

It does not seem to make a difference at harder throttle.

It is only a light noise. So maybe:
1. Lifter/tappet noise (I thought this)
2. Loose piston (I hope not!)
3. Loose connecting rod (I would have to check this.. have to change the leaking gaskets anyway)
4. Detonation (Feel silly.. what is this?)

Thanks again everyone for your input.

Detonation is spark knock or 'fuel knock', also typically called 'ping.' It is a light, irregular 'rattley' sound caused by low-octane fuel or by spark timing that is too far advanced. It shows up mostly during acceleration. From your description, I think you can discount that possibility for now. Although the possibility is not out of the question.

Since the knock in question seems more noticeable with the warm engine, I would suspect a valve lifter, or some other oil-related condition. What happens if you shift to neutral and let the engine idle when you hear the noise while on the road? Noise disappears, continues? If it disappears with the transmission in neutral, can you bring it back by changing engine speed- transmission still in neutral?

At this point, I don't think you have a piston or connecting rod matter. A connecting rod remains a possibility, but you should be able to hear that if you speed up the engine gently to a couple thousand RPM and release the throttle slightly. In the very early stage of rod bearing wear, a knock tends to show up just as the load decreases as you release the throttle.

Hope this helps. Keep in touch. I'm sure others here will have more ideas as well.

Harvester

Hi Harvester...

Thank you again for your information and help. I still do not know what the problem is but I have additional information:

1. I am getting ready to work on the cab floor and removed the entire padding etc from the floor. Now you can hear the tapping other times besides on the Highway.

2. When the engine is cold, the tapping sounds tinny. It slowly turns into a tap as the engine warms up.

3. You do not hear the noise from the top of the engine when the hood is open. All you hear is the engine.

4. You can hear the noise if you go under the truck or from insiude the cab now that the padding is removed.

5. On the road, I can hear the tap when I am in 4th doing 20 to 30 MPH. Because the rest of the truck noises are low, I can hear the nose better.

6. I do have a separate "knock" that I would refer to as the fuel knock. This noise usually appears for a second or two at harder throttle.

I plan to try disconnecting the vacuum line to see if the noise changes but wanted to pass this information on first. I fear this sounds like a lower end engine problem. It is a shame the truck only has 28,000 miles on it.

..Russ

Maybe a wrist pin?

I guess a lifter is weak and collapsing.

Ok, more checks and questions:

1. Since you hear this noise more from inside the cab, put the transmision in gear and hold the clutch. Can you hear the noise? Yes, the noise is more likely in the engine, clutch pressure plate or pilot bushing; No, the noise is more likely in the transmission or clutch disc. Do this a few times to see if the results are predictable.

2. When you hear the noise while in 4th gear (vehicle moving, I assume), does it change if you shift to neutral and let the engine idle? Then, is there any change if you engage/release the clutch with the transmission still in neutral?

3. If you still hear the noise during step 2, clutch released/pedal depressed, what happens if you raise the engine speed slightly? No change, no engine noise, transmission is suspect; change in sound with engine speed, engine-related condition.

4. If you have no noise during step 2, (coasting, transmission in neutral, no change with engine speed) shift back to 4th gear and release the clutch. Does the noise return? Yes, more likely transmission-related. No, does the noise return with a change in vehicle speed (you probably have slowed some)?

I think this progression all makes sense, but if there is a contradiction or a failure in the logic, hopefully someone else will point out the failure and set the matter straight.

Some words of caution are in order here: don't convince yourself that you have piston or connecting rod trouble at this point; the evidence for those possiblilites is not sufficient yet. Those conditions are quite rare in an engine with low milage, and there are too many other possibilties to eliminate before settling on those possibilites.

If the evidence continues to support connection rod trouble, I would start by removing the oil pan and checking rod clearances with plasti-gauge. If you find a worn bearing and the crankshaft is not damaged, ok, replace the bearing, but understand that fluid dynamics will prevail, and if you tighten one bearing in the group, the others will likely leak more oil and cause pre-mature failure of the one you just replaced. Then you have a more serious condition to deal with. So, if you find a loose bearing and no crankshaft damage, replace all the connecting rod bearings and make sure they all have the permissible clearance. That is cheap insurance.

As for a loose piston or wrist pin, maybe, but those are way down the list of possibilites.

So the summary of this last advice is: don't convince yourself that you have a problem that doesn't exist.

Keep in touch.

Harvester

Hi Harvester...

Here are my findings to your latest questions:

1. I checked and the noise is from the engine and not the transmission.

2. The noise changes with the RPMs of the engine. Since I am in 4th and only going about 25mph, the rest of the truck and road noise is at a minimal which makes it easier to hear the tap.

3. The noise changes with the RPMs of the engine.

4. At this point we have determined it to be in the engine.

Over the weekend while at my local car club's show and swap meet, a fellow member pointed me in the direction of a local shop that works on older vehicles. They took a listen to the truck and felt the noise was a 'lazy' lifter. They felt the truck ran very strong and try not to over 'hear' the engine. It would be normal for a 43 year old engine to make a little noise even at 28,000 miles.

This sounds like this was one of the results listed above in this form.

So for now, I will wait and keep my ear peeled for any changes but I wanted to thank you and everyone else for helping me with this issue. If I had let my mind go nuts about it, I might have just left the truck on the side of the house in fear of causing too much damage. As it is, with this economy, things are tight and I really could not afford a major engine problem. So thanks guys… this is really a big load off my mind!

Thanks... Russ

A 66 C30 with only 28k miles and the seller says to use 20w50 ???? I would have to see it, but I have severe doubts. Even if clean original, it is more likely 128k.
The 327 was very happy with 30w even though a gauge would typically show very low idle oil pressure.

I can see a low mileage engine in a larger truck. Some of them really didn't get used much, but probably spent a lot of time idling too. I think I would still use 10w-30 or 30w in the chevy small blocks. A heavier oil may mask a noise problem slightly, but doesn't solve the problem.

I think you are on the right track and I'd start with readjusting the lifters and go from there. Get a cheap stethescope for listening to your engine. You will hear the before and after sound when you make the adjustments. You will remember that sound and use that information down the road too.

Hey Builder...

Thanks for the information. I was thinking of using lower weight oil. I used to use 10w40 and 20w50 on my high mileage engines (Had a 76 Pontiac with 265,000)… but I am willing to try 10w30. Thanks for the advice.


Hey Richard...

I bought the truck from the 3rd owner. He put 20w50 in everything he ran so I followed through thinking it was 128,000 miles. But as stated above.. maybe 10w30 will work fine.

The truck sat at a gas station in Brooklyn most of its life. I think they realized they bought the wrong tow setup as the bar could only be used on solid bumpers found mostly on the trucks and some of the older cars. The second owner used it to pull a trailer once a week to New York City from Queens for an Antique and craft show. The 3rd owner bought it thinking it would do well towing his race car out to Riverhead Raceway in the Summer but soon found out a ramp truck worked better.

I didn't put much thought into the 3rd owner stating the mileage was original until I started working on her. She had her original shocks, the brakes look like they were only done once, the inside of the valve cover's were clean and also the intake manifold was spotless when I rebuilt the carb! The gas station seemed to keep everything greased up including grease on the metal brake and gas lines running up the frame. Trust me, I know what a 128,000 mile truck drives like and how it looks inside. This is not one of them.

UPDATE: Just in case someone else runs into this, I thought I would state what I found:

The tap noise seems to have disappeared with the change in the oil weight. I am using 10w40 instead of the 20w50 now.

There was another annoying noise that I could not figure out. It turns out to be the timing chain. Now that the tappaing is not disguising any noises, you can hear that it is the timing chain. I was told it not an issue to worry about. The truck has less than 30,000 miles on it.

Now that my engine fears are gone, it is time for a louder exhaust! Thanks again everyone for your help!!

I wouldn't dismiss the fears just yet. one good reason to run 20w50 is to disguise engine noise in a vehicle you want to sell, case in point the 85 Jeep my son bought a few years ago. It started knocking right after changing the oil to 10w40. A timing chain slap indicates to me more than 30k miles.

Hey FriedGreenTmater

When I ran the 20w50, I had the tapping noise. Now I run 10W40 and the tapping has gone away. Nothing to hide there. As far as the chain slap, I was told it is common on these engines, especially if the engine is original with less than 30,000 and has never been rebuilt. This came from a group of guys in their 70's who use to work in a shop back in the 60s and 70s.

I know, everyone is always out to say there is no way this truck has less than 30,000. But think about it, it is a local tow truck that spent most of its life sitting at a gas station, used occasionally whenever they needed a tow.

I have pulled the carb to rebuild and was amazed how clean the intake manifold throat was. When I pulled the valve covers off to replace the leaking gaskets, the inside was almost spotless. You could still see the orange dots on each rocker left from inspection at the factory. I could list many more examples from the rest of the truck but believe me; I have a 1966 truck with less than 30,000 miles.

Thanks again for the info.

These old big bolts can have low miles on them. I have a 1949 grain truck that only has 57,000 miles on it.