Symptoms:
The transmission on my 5 speed 2000 Legacy GT sounds like an angle grinder or go-cart (grinding/buzzing/putting noise). Started making noise overnight, in every gear and in neutral, sounds like something rotating/grinding and is proportional to the car's speed. When the car isn't moving there isn't any sound.
Another interesting thing is that in 5th gear (possibly others, but most noticeable in 5th) it makes noise with 0 throttle, less noise with 1/8th throttle, and then more noise again if you go past that.
What more knowledgeable friends have said:
A trusted mechanic said it's probably a bearing in the transmission, but he doesn't work on them so I'm not confident he knows the exact issue.
A good friend said he had a similar issue on his 2000s era Forester, and it was the center differential, but I've heard that if the differential is the issue, the symptoms will get worse while making tight turns, and I haven't experienced that, at least at low speeds.
Possibly relevant info:
I had a rear tire replaced this past summer, they didn't shave it down. I measured them and it was essentially the same size. Maybe a small difference in diameter caused an issue?
185k miles
