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