Humm in the rear? At certain speeds

[bikerguy71]

Hello- I just purchased a 1989 325xi 4 door automatic with 126,000 miles. Overall a nice vehicle that has reciepts showing that it has been maintained well. Runs and drives great, but a certain speeds I hear a loud humm coming from the rear during acceleration. If I let off the gas, the sound goes away...very strange. Any ideas?

[zedfez]

Rear diff?

[bikerguy71]

Thats what I am thinking to. The noise(hummmmm) is louder under power, and is coming from the rear of the car. I let off the gas, and the humming sound is quieter. Drives me nuts. Is there something I can do to determine the what it is?

[zedfez]

Honestly, I don't have enough experience with diff's to know. It might be worth checking the fluid for metal bits, or trying to take a look through the filler hole and see if anything is obviously wrong.

You could also try jacking the rear end up and having someone rev the engine while you use a stethoscope or metal rod against the diff to see if that is where the hum is coming from. Just make sure to chock the front wheels and use good jackstands so the car doesn't run you over.

[bikerguy71]

I drained the fluid and actually look really good. I stuck my finger in the drain hole to see if I could feel any shavings or crud, but it was absoutely clean. I refilled with 2qts of Royal Purple 75-90 and drove the car around. The hum is still there:(. Is it ok to jack the car up and run the wheels unsprun? That worries me a little, but if everyone thinks that is ok, I am willing to try. I have a 4 post lift in my garage, so that wouldn't be difficult for me to do. Does everyone agree that is ok to do? I would really like to pin this down. I would like to dry this car in the winter as I live in Michigan, so I have a little time to play with it.

[Need4Speed1299]

Do it.

[bikerguy71]

So I drove up the car on my hoist. Jacked up all 4 wheels and had my fiance drive the car while I listened to the differential as she gave it gas. At first I didn't hear much, but as the tires sped up and it shifted into 2nd gear I could start hearing the noise or humm. It appears that it is coming from the front of the differential where it meets the drive shaft. I think that it is either the front bearing in the differental where the driveshaft meets or the u-joint a few inches in front of the differential. Do any of you know an easy way to tell, or should I just have the bearing and u-joint replaced? Last question, is it possible to replace the u-joint and front bearing without removing the differential. Should I take to a shop to have fixed or is it something I could handle without too much effort? Do you think that this should fix it. I am pretty sold, but would like input from you pros before I move forward.

[bikerguy71]

All right people....I really need some advice here. I replaced the rear differential with a low mileage one. I replace it and no change to the humming sound from the rear. I thought for sure that was it, but no such luck. While replacing it, I noticed that the Ujoint on the drive shaft had a flat spot in it. Made me wonder if that could be it. Did some searching and asked some experts in drivelines.....they said that it could be the source of a humm. I figured why not. Bought a remanufactured one online and just got it in and replaced it tonight. Sad to say, that I have failed once again. Now I am completely confused. Took it for a ride and the hum is exactly the same. It seems like the sound is coming from the rear driver side. What can it be? I took the CV out and took it for a little spin....no change. So it can't be the CV...at least the left one anyway. Could it be the wheel bearing? It does not seem to change pitch while turning left and right. It is at it loudest at approx 55 under load. If I let off the gas, it quiets down....but I can still hear it faintly. Once I hit about 60 it tones down again. Any ideas??? I am ready to take a video and post on Youtube. Maybe one of you out there can help out a frustrated member. I have $850 into fixing this and a lot of hours and no success. It is now a mission....but I am scared to spend any more money. What could it be?

[gw ix]

Rear wheel bearings, I had the same issue on my e46 did the rear wheel bearings (yes do them both) and no more hummm. My humm would start at about 35mph and go away at 70mph and then return around 90mph.

[ieatpeople]

i had a hum and occasionally a grinding noise from the rear diff in my 87 "is". turns out that the lsd needed a friction modifier additive. once i put it in, it was quiet.

[Holland]

I was going to say pinion bearings but you swapped diffs so I would go with the wheel bearing but that is my guess.

[supernaughtIX]

wheel bearing. Tires in good shape? trailing arm bushings ok?
Speaker HUM

[Leaf]

Rear wheel bearings. The humming should increase when taking a corner hard (weight being put on the bearings). Its a pain in the ass to do but it's really not incredibly difficult. Probably a full days work for an iX or about $100 to $150 per axle in labor. I think the bearings are like 40 bucks a piece.

[bikerguy71]

Well I replaced the wheel bearing(what a pain) and it didn't get rid of the hum. The hum peaks around 30mph and again at appox 50. Over 60 I get a little vibration in the rear. I have now replaced the drive shaft, rear diff, and wheel bearing. CV seem fine. I am out of ideas. The humm is loud. I put the car in neutral, and even shut the engine off....noise is still there. Don't know what to do now. The car also has new brakes, tires and muffler.

[supernaughtIX]

has it happened only since the brake job? maybe something has come loose?