Chasing a humming/whining drivetrain sound

[tbolt1003]

I have a 95 Impreza, 2.2, auto, 135,000 miles.

 

About 2 months ago, the car developed an intermittent whine and humming sound from what initially sounded like the rear of the car. Fast forward to the last few days and it's now hard to tell if it's coming from the rear or the center of the car. Initially, I thought wheel bearing, so when the car was in for its annual inspection a few weeks ago, I mentioned the hum to my mechanic and they said they'd look at it. After (2) individual mechanics looked at it, they cannot find where the hum is coming from, although they did say my rear hub bearings and the rear differential are fine.

 

The sound starts off as a very low hum at low speeds but gradually increases in pitch as speed increases. I'd say it begins to be noticable around 15-20mph and only can be heard on acceleration or when I'm inbetween throttle/no throttle. Sometimes the hum is very ear-piercing at highway speeds and I have no choice but to let off the gas.

 

The gear oil in the differentials is clean and has no burnt smell to it, both diffs at full mark. The car drives fine, no vibrations or shaking felt, hum pitch does not change as suspension moves (I purposely drove down a bumpy and hilly road to rule that out).

 

I read about the center support bearing on the rear driveshaft going bad and causing a humming sound. I crawled under the car and worked the driveshaft around. There is definitely a fair amount of play. I can move the driveshaft all around, up and down, side to side a good inch. What is acceptable play for this bearing? Could this be the culprit?

 

One final note: when I put a fuse in the FWD holder under the hood and drive the car, the noise disappears completely, so it's something from the rear driveshaft back, I'm assuming, correct? Any thoughts? Thanks!

[Fairtax4me]

Probably be a bad u-joint in the driveshaft. The u joints on these are tiny and don't exhibit much play when they fail, but they make a hell of a racket.

 

The center support bushing will move around quite a bit but it's not easy to test the actual bearing without removing the driveshaft.

[tbolt1003]

Thanks, Fairtax. I was hoping you'd be one of the people to respond. I've did some searches for the center support bushing and I can get a completely new driveshaft with new center support bushing, new front yoke, new rear flange and greasable u-joints for $385, no core charge. I thought of going to a local salvage yard and pulling a shaft, but I won't know if it's good or not until I get it in. With a new unit, I'd be killing multiple options in one shot, ie bad joints, bad center bearing, etc.

[tbolt1003]

Thanks, Fairtax. I was hoping you'd be one of the people to respond. I've did some searches for the center support bushing and I can get a completely new driveshaft with new center support bushing, new front yoke, new rear flange and greasable u-joints for $385, no core charge. I thought of going to a local salvage yard and pulling a shaft, but I won't know if it's good or not until I get it in. With a new unit, I'd be killing multiple options in one shot, ie bad joints, bad center bearing, etc.

[Fairtax4me]

It's pretty straightforward once the driveshaft is off. Notchy or sandy sounds when spinning the center bearing would indicate that its bad.

The u joints you can check with it still on the car. Try to twist the yoke opposite of the tube to see if there is any play. It helps to have a couple of large channel lock pliers.

[tbolt1003]

Thanks. I'll crawl under tomorrow and see what I can find.Taking the shaft out does look very simple, indeed!

[tbolt1003]

I crawled under the car today. With the car in park and the emergency brake on, I could not feel any looseness between the yoke and U-joint, nor between the flange and joint. The shaft did spin slightly, perhaps 1" of free play before the trans and/or differential prevented it from turning any further. Should there be any free movement of the shaft? In the little bit the shaft was able to spin, I did hear some sand-like grinding around the center of the shaft, which I'm assuming is the carrier bearing and it just didn't seem "right" to me.