(updated with the answer)

Symptoms:

The transmission on my 5 speed 2000 Legacy GT (AWD) 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 noise.

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?

ANSWER:

I'm transcribing from the mechanic so this may not be 100% accurate:

The transfer gear was split, which damaged/destroyed center differential (and the back clip came off). The transfer case damaged as well.

The mechanic said it was just normal wear and tear, so I'm not sure if this had anything to do with one of the tires being replaced.

Edited December 18, 20196 yr by consmi
answer

I have not worked on a Subaru manual tranny yet, but I have done a lot of older 4 and rear wheel drive trannys.

Does the sound change when you press and release the clutch without the car moving? (clutch release bearing)

Does the sound stop when you hold the clutch in and put it in gear with the brake on? (input shaft bearing or something further inside)

I'm sure someone that has worked on them will chime in. They will want to know the answers to those questions to start with.

On 11/27/2019 at 5:00 PM, Rampage said:

I have not worked on a Subaru manual tranny yet, but I have done a lot of older 4 and rear wheel drive trannys.

Does the sound change when you press and release the clutch without the car moving? (clutch release bearing)

Does the sound stop when you hold the clutch in and put it in gear with the brake on? (input shaft bearing or something further inside)

I'm sure someone that has worked on them will chime in. They will want to know the answers to those questions to start with.

When the car isn't moving there isn't any sound, I realized my initial post wasn't clear.

I'm guessing it is all wheel drive. Have you checked the oil in the front and rear differentials and the transmission?

Get the wheels off the ground and run it in gear and locate the area of the noise.

Front differential, transmission, rear differential, driveshaft u joints, support bearing in the middle of the driveshaft. Subaru calls the driveshaft a propeller shaft.

was any work done to the car just before this occurred?

I’d say it’s the bearing between 4th and 5th on the upper shaft. 

Probably caused by low oil.

It’d be cool to pull the box and get some pics of it’s internals. 

Cheers 

Bennie

On 11/29/2019 at 1:29 PM, Rampage said:

I'm guessing it is all wheel drive. Have you checked the oil in the front and rear differentials and the transmission?

Get the wheels off the ground and run it in gear and locate the area of the noise.

Front differential, transmission, rear differential, driveshaft u joints, support bearing in the middle of the driveshaft. Subaru calls the driveshaft a propeller shaft.

Yes it's all wheel drive, transmission oil is fine. Have not checked differentials.

Good idea, I need to get more jacks.

Thanks

On 11/30/2019 at 12:03 PM, 1 Lucky Texan said:

was any work done to the car just before this occurred?

Brake rotors and pads replaced, handbrake didn't work so we were leaving it in reverse gear to hill hold while parking it for about 2 weeks before going back to get it tightened.

Harbor Freight has good jack stands.

4 hours ago, consmi said:

Yes it's all wheel drive, transmission oil is fine. Have not checked differentials.

Good idea, I need to get more jacks.

Thanks

no - you need to get jack stands.. You should not be crawling under a car that is held up with just a jack. Jacks fail, far more often than people think. Jack stands are far safer.