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This isnt a legally binding, foolproof way of diagnosing gearbox noise but at work this is how we do it and after hundreds of years of being a mechanic its pretty accurate.
Firstly you need to isolate the noise. Is it 100% from the gearbox? Remember your car has a seperate transfer box and 2 differentials.
Its tempting to jump to suspecting 4th and 5th gear but remember your road speed is relatively high when in these gears. Could this noise be from something other than the gearbox at high speed rather than a noise from these gears? Release bearings on your clutch, fuel pumps, fan belts, timing belts and air intake pipes can all whine so make sure it is 100% the gearbox. I have to say this, not because I doubt your hearing but because if you are goint to the trouble of rebuilding a gearbox you need to know you are going to fix it!
Next, the load test. Is the noise worse on acceleration or over run? Noise on acceleration tends to be bearing noise, on the over run tends to be worn gear teeth. Bearings can groan as well as whine as can gears. Bearings cheap, gears not so!
Next, check the oil. We would typically drain out the oil and filter it. An oil filter box with a bit of blue roll is ideal. Drain the oil thru it into a drain tray or similar. Are there any bits? Gold or brown oil=ok. Grey suggests something has worn and disintegrated. Chunks of metal means something has failed quickly. Gold/metallic means a synchro has broken up.
Grey usually means a bearing has failed allowing something to move and put more load on something else.
Generally speaking if a bearing has failed you would routinely replace all the bearings on the geartrain. Assuming all the bearings are the same age, if one has failed the others wont be far behind.You wouldnt want to change the mainshaft bearings only to have to remove it to replace the idler shaft bearings the following week!
Gear wear tends to be caused by driver error. Every Subaru (and several Kia/Hyundai) gearbox I have had to rebuild has been because the driver has been driving with their hand on the gearstick! When you do this it pushes the gears together so the thin film of oil between gears is squeezed out. This causes the gears to heat up. Hot gears become harder and wear everything they contact faster. Not good!
Its not enough to batter in and change the broken bits, you need to understand what caused them to fail.
I'm really curious as to how you jumped from rear wheel bearings to gearbox related, and I guess that is tied in to what made you think rear wheel bearings in the first place.
Rear wheel bearing noises are going to be road speed related, regardless of which gear you're in, gearbox noises tend to be related to which gear you're in.
It's also, at least in my experience, a very different sound, rear wheel/axles are usually a lower pitched hum whilst gearboxes are a higher pitched whine., the difference being caused by the rotational speeds.
Good question - I used an electronic stethoscope on various parts under the vehicle (gearbox/transfer box/diffs/axles) whilst driving to try to locate the noise and there seemed to be a louder noise at the rear wheel bearings which, being relatively quick and easy to fix, made sense to be the first thing to try!
To be honest it may have been part of the noise! but clearly there is an issue remaining even after replacing these - which alongside the known R72 gearbox noises leads me to believe that is probably the problem...
but clearly there is an issue remaining even after replacing these - which alongside the known R72 gearbox noises leads me to believe that is probably the problem...