Please help diagnose driveline noise

[JHZR2]

Hi,

I posted the other day regarding LSD chatter and groans at startup in my 91 BMW 318i. I think Ive figured them out, theyre due to the muffler mount actually. Ill just need to re-adjust them.

However, I still have a little whine and a funny occurrence. the issues are:

1) I get a 'woooo' noise, particularly at ~63-67 MPH. It isnt very bad, not very noticable. However, if I let off the throttle completely, especially at 67 or so, I get a really loud 'wooooo' noise, on top of the one that I already heard. Thing is, if I press in the clutch at that point, the loud one goes away. The other quieter one still remains. Oddly enough, if I go under 52 mph, or over 67 MPH, both sounds go away completely! The car is FAR quieter at 70 MPH than it is at 65. Problem is, most roads I travel on have an actual flow-of-traffic speed of ~65 MPH. (BTW, My car had redline MTL before the service, D4 atf after. I just swapped back to fresh MTL to see if it would make the noice go away, but it doesnt, so it was not a function of changing the fluid to a thinner one).

2) I get a clunk when I change gears, or especially if going backwards. I had the car up on a lift today to re-check, and there was nothing loose. The driveshaft moves maybe 1/8 of an inch before the output shafts turn. I looked at my old diff, and there is no rotational play before the output chafts begin to turn. However, its apples to oranges, as the old one is off the car, the rebuilt is installed.

Any advice or info would be appreciated. I apologize for posting questions like these numerous times, but as I figure bit by bit what the noises are individually, Id appreciate help on getting rid of them. The 'woooo' noise, especially when off throttle and it gets real loud is my biggest concern, really because of annoyance when driving long distances in the car. Any advice info or help would be great. My next step may be to try Lucas 85w-140 in my diff, as it claims to help reduce whine and noise.

Sorry for the long post and thanks again for your help.

JMH

P.S. For those who didnt read my other posts, the car background is as follows:

91 BMW 318i, 98k miles, 1.8L 4 cyl engine, 4.10 LSD, MT. My old diff whined badly; It would make a high pitched whine when loaded, and be silent when it was unloaded. I got a very good BMW shop to rebuild me a different diff, and had it installed. The whine I just explained is gone. While having the diff swapped, I replaced the rear subframe mounts as well. All parts OE BMW. I had M1 75w-90 put in the rear (per the rebuilder's reccomendations), and Redline D4 ATF put in the trans.

 

[JAYCEE]

Clunks are sometimes caused by a lack of grease on slide yokes. I don't know if your car has any of these.

Grease is the first thing I'd check though.

 

[Shannow]

quote:

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Originally posted by JAYCEE:
Clunks are sometimes caused by a lack of grease on slide yokes. I don't know if your car has any of these.

Grease is the first thing I'd check though.

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Ahhhhh.....4Runner, clunk/graunch on take-off.

Thankyou.

 

[ikeepmychevytoo]

"wooo" noise at some particular speed is probably what is referred as "boom effect" on acoustics. Every chamber (car's body on this one) has a specific "natural frequency" and all neearing frequencies might cause resonance resulting in an -irritating to ear- highly amplificated noise.

If it is due to the exhaust, using original equipment silencers, mufflers, also the hangers is crucial (are the rubber mounts hardened...). If it is the diffrential the mounts come to my mind on every independent rear susp. car.
Are the "subframe mounts" include the differential to subframe (or to frame) mounts too... if they are replaced are they torqued to the specifications... They can be very specific, as on my Chevy "when trailing arm at this angle it should be torqued to that".

 

[1 FMF]

the 65mph whine is probably the differential ring & pinion, which is normal. At a certain speed, or rpm of the gear turning, you'll get harmonics. Mine also happens at 65, with just enough pedal to have no load on the drivetrain is when it's loudest. If I take my foot off the gas completely, or go +/- 2mph off 65 the whine goes almost completely away, I say almost completely because camaro's are not known for their quiet ride to begin with. Add this to specific body styles, lack or poor placement of sound deading materials and you'll hear the gear whine. Solution: just go faster and turn up radio

mine also clunks when shifting or going into gear (manual 6-speed) but I figured that was because it's a chevy.