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
