last auto noise question - sound under throttle vs. engine braking

[JHZR2]

Hi,

As many know, I have a 1991 BMW 318i. It has a 4.10 LSD and MT. Its a great running and driving car, and gets me good fuel economy too.

when driving 65-68 mph, if I take my foot off the throttle, I get a very loud 'woooooo' noise coming from the rear of the car. Ive tried it in 4th and 5th gear, it happens the same.

It is terribly annoying, as the driving I do often has me going around those speeds, and often I need to take my foot off the throttle and reduce speed a bit.

I know for certain that if I let off of the throttle, it starts to 'woooo' and then I press in the clutch or pull the trans out of gear, the noise goes right away. So maybe its the unloading of the diff at that speed? Maybe its the transmission, but it seems that it is the exhaust. I think this because if I put a screwdriver to the tailpipe and to my ear, I can hear resonances that are similar in sound to what I hear off throttle at 67 mph. I cant hear them at idle, without the screwdriver.

So, my last noise question is: what is the difference in exhaust sound (and I guess also intake sound) between loaded conditions and off throttle 'engine braking' conditions, when I just take my foot off the pedal, car in gear, at speed. Is there some sort of significant change in flow that may cause a weird resonance in the exhaust system?

Any advice would be appreciated. I apologize for the many long posts, but look forward to any replies.

JMH

 

[Quest]

check your exhaust pipe for holes due to corrosion. There may be a resonant frequency under certain RPM where the "woo" sound would suddenly becomes very significant.

when car is off gear during high speed coasting , your RPM drops off and left that "resonant freqency at certain RPM range" usually down to 1300rpm or similar and that's when the "wooo" goes away.

Have patience my friend.

 

[JHZR2]

Thanks... Ive tried nearly plugging the exhaust, to see if there were any leaks. I checked the welds where they put in the new catalytic converter, and there is no sign. The mufflers are also fine.

The funny thing about the resonances is that it not really engine dependant, but road speed dependant. going through the 67-65 mph in 5th gear at 2750 or 4th gear at 3500 RPM yields the same noise. This is the one reason why I have to question if it is the exhaust itself. Since its road speed dependant, I have to think it to be driveline dependant, but pressing in the clutch makes it go away, and nothing was done to the transmission, only the diff. I take it that it couldnt be the diff, or the trans output bearings. Maybe the transmission input or gears themselves? It would have to be a common item, since Ive replicated this in 4th and 5th gear.

If I take the engine out of gear, go along at the speed where the woooo shows up, and rev the engine (out of gear) through the range, it is not replicated. However, the mass flow throuh the exhaust would be vastly different, I assume, revved to the range vs. being pumped in gear at that RPM range.

I can get similar, less loud resonances by reving out of gear at different speeds. The sound is very similar, though MUCH quieter, so its hard to tell.

I just cant get over how if it was a driveline resonance, how it just showed up when I replaced the diff, and if its an exhaust resonance, how it only occurs at a specific road speed, regardless of engine RPM, and I assume, as a result different flowrates through the system.

Thanks again for your help,

JMH

[ June 29, 2005, 09:41 PM: Message edited by: JHZR2 ]

 

[JHZR2]

I have ben advised that it may be the pinion bearing... it apparently will whine under no load situations.

So I went from a diff that whined under load to one that whines when not under load.

I have to guess that this is the troublemaker...

Maybe itll get right sooner or later.

JMH