328is 3.91 lsd faint hum at about 50 mph from rear of car

[blanko]

Hey all. Searched around and found a few related threads, but still wanted your guys' opinion about my specific issue.

I just bought my second 328is a couple weeks ago, and its awesome, but I am hearing a noise that concerns me a little. When I am driving at a steady 50-53 mph I can hear a faint hum coming from the back of the car. No grinding, vibration, or clunking sound just a little hum when cruising at that speed. I don't hear it at 40 or 60, or any other speed at all...and If I hit the gas just a tiny bit the noise goes away until I let off and cruise again.

First thing that came to my mind was the diff, because I am pretty sure its older than the rest of my car. Its a 3.91 lsd out of another other BMW, but I don't know exactly where it came from. Wheel bearings also came to mind, but I would expect to hear that all the time, not just at certain speed. What do you guys think I should look into?

[blanko]

bump

anyone have a clue?

[E36 328is]

Try changing the diff fluid and see if that helps. How is the accelaration with that 3.91?

[blanko]

Try changing the diff fluid and see if that helps.
yea. thats probably what i will try first.

The acceleration gain w/ the 3.91 is pretty substantial. The car feels extremely torquey and can launch about as quick as most AWD cars, assuming they don't dump the clutch at 6k or something. I'm still new to this car so i cant really give you an amazing description, but I did own another 328 before this one with the stock 2.93 open, and the difference is night and day. First gear is ridiculous. reminds me a bit of a friends trans am ws6. It is definitely a trade off though. The car tops out just under 130, and is around 4k rpm at 80 mph

[E36 328is]

That is plenty fast enough for me in terms of top speed I never go above 120mph anymore. Do you find the RPM's on the freeway anoying or not that bad? I am really contemplating this diff since I never cruise above 75 on the freeway since there is so much traffic and I love getting up to speed as fast as possible.

[seanpuck]

I have the same diff in my very slightly modded (m50 manifold,cai,and software) 97 328i and first gear will snap your neck but you do lose alot in mpg -I drive pretty fast and average about 21. Car also tops out at 129 (probably enough) for me and my 11 year old car.

[E36 328is]

I have the same diff in my very slightly modded (m50 manifold,cai,and software) 97 328i and first gear will snap your neck but you do lose alot in mpg -I drive pretty fast and average about 21. Car also tops out at 129 (probably enough) for me and my 11 year old car.

Do you like the diff? Or do you wish you went with something less aggresive? Have you ever raced a m3 or other cars with it?

[jmo69]

It could just be tire noise, swap the tires front to back and try again. Just my $.02

[97'scort]

I have the same hum. Seems pretty common on these cars. It can be a few things: input bearing in the diff; driveshaft main bearing; rear wheel bearings.

[seanpuck]

Do you like the diff? Or do you wish you went with something less aggresive? Have you ever raced a m3 or other cars with it?

Yes I'm happy with how quick my car is. Night and day between mine and a stock 328i. I've done alot of stop-light races and can pretty much keep up or at least come close to a stock e36 m3 up to about 120-lucky for me not too many want to go much faster than that . I raced a e46 m3 and he smoked me . Usually I only take on cars that I think I can beat .

[blanko]

It could just be tire noise, swap the tires front to back and try again. Just my $.02
I'll give that a shot.

I really like the diff on the streets. It takes off damn quick from a dead stop. In fact last night when me and a buddy in his evo 8 were cruising back home, we pulled up a to 99-2000ish civic si, and the kid starts revvin the crap outta his engine and was inching forward ready to go on the green. I took off so much faster than him I actually thought he didn't even take off hard. My friend in the evo confirmed that the civic kid did indeed race. He let the kid go for a few seconds and then proceeded to fly by him with ease. i shut down at 90 and just cruised 55-60 the rest of the way down the street, and the kid kept his distance from me..probably out of embarassment. I think the evo and I would be a pretty good race assuming I launch good. I know I would probably lose, but i don't think by a ton.

Anyway, the 3.91 is sweet on the side streets but it kind of bugs me being over 4k rpm at 80. As long as you go below 75 on the highway it isn't bad at all.

[97e36convert]

Probably just the new diff making some different noises, my gears in the dodge sound like a jet engine when I slow down. If you're really worried, fluid would be the best place to start.

[blanko]

Probably just the new diff making some different noises, my gears in the dodge sound like a jet engine when I slow down. If you're really worried, fluid would be the best place to start.

what fluid would you recommend? BMW stuff only? Redline?

[blanko]

I just found this description of worn output bearings after a google search. It fits exactly with what i am experiencing.

"A BMW differential's most common wear item are the output bearings. Output bearing wear is characterized by a hum or whine from the back of the car that will become audible between 3000-4000 rpm, usually more noticealbe in fifth gear at a constant cruising speed. The noise will stop or change when you lift your foot from the accelerator pedal and may also change as the suspension rebounds from dips and valleys in the roadway. The perceptability of the noise is dependent on the load on the bearing The noise may also stop entirely above a given rpm. Also, many cars with differentials showing this wear have gone another 50,000 miles with this noise and no further, so it's not necessarily something that needs to be addressed right away."

heres the link if interested (http://bmwnut.blogspot.com/2006/03/bmw-differentials-part-ii-diagnosing.html)

[328is96]

I have a 96 328is with the 97 m3 3.23 diff and it made a big difference, i have raced a 93 5 speed m3 and a 97 m3 and i was able to beat them both but of course i have other mods besides the diff.

[1rstbmw!]

I have a 96 328is with the 97 m3 3.23 diff and it made a big difference, i have raced a 92 5 speed m3 and a 97 m3 and i was able to beat them both but of course i have other mods besides the diff.

92 M3?:confused

[billygoat777]

I just found this description of worn output bearings after a google search. It fits exactly with what i am experiencing.

"A BMW differential's most common wear item are the output bearings. Output bearing wear is characterized by a hum or whine from the back of the car that will become audible between 3000-4000 rpm, usually more noticealbe in fifth gear at a constant cruising speed. The noise will stop or change when you lift your foot from the accelerator pedal and may also change as the suspension rebounds from dips and valleys in the roadway. The perceptability of the noise is dependent on the load on the bearing The noise may also stop entirely above a given rpm. Also, many cars with differentials showing this wear have gone another 50,000 miles with this noise and no further, so it's not necessarily something that needs to be addressed right away."

heres the link if interested (http://bmwnut.blogspot.com/2006/03/bmw-differentials-part-ii-diagnosing.html)

When I bought my e34 my diff was making this noise, the internals were shot because the bearing had been bad for so long. BUUUUT it meant upgrade to m5 lsd :D

[ekib]

FWIW, I have the same noise. After changing the diff fluid to Royal Purple, the noise has gotten a bit worse. Can this output bearing be replaced easily or is it easier to just but another used diff? Mine is open so probably quite a bit cheaper than lsd...

[nyboriboy]

my car just started doing the same thing and i thought the diff needed fluid...this just gives me a reason to get a 3.91 lsd :buttrock