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Hi,
From what you are describing to me it sounds like it’s a main input shaft bearing.
When you have the clutch engaged and the transmission in neutral, it is spinning only that shaft.
When you are using only the fourth gear you are running the output shaft at the same speed as the input or closer to a one to one ratio.
This overall selection is Slower than when you are in lower gears building up a torque advantage. When you shift down there is a counter shaft that is picking up speed as the gear diameters change in size and therefore speed.
Pressure shifts through the teeth engagements and the thrust of pushing the car is transmitted through the lengths of the shaft. The bearings are having to hold them in place.
When you are in neutral, the wear around of the rolling elements of the bearings are moving around.
When this happens the gears teeth are also moving back and forth and in and out of engagement. This is due to some radial play of the bearing as the two shafts are not held in parallel.
In many cases it’s the first or most outer bearing. It’s the largest, spins at engine speed almost all the time and is usually the loudest because it closest to, guess what, a bell housing! Might as well be a audio speaker cone.
Basically, You have a lot of miles on this transmission!
A little excess play is everywhere. It doesn’t get better with things shifting or sliding sideways.
Put in New bearings and getting the lateral play corrected with shims and it should stop it.
How does it shift from one gear to another ... smoothly?
Is the oil in there fairly free of metal ... brass or steel sparkles ... during recent oil changes?
You might have to consider the synchro’s or another transmission with less miles.
If this one is good, then keeping what you know! A new bearing, here or there, might be better than trusting some ship-shod transmission shister.
It seems that your mechanic did you a good job! He did all the right things back 3 years ago.
Questions are, was he reasonable?
Does he know someone?
Phil
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