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  #1  
Old 12-18-2011, 05:29 PM
GGR GGR is offline
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woom woom woom woom noise while cruising at 80 mph

Hi all,

this is on a W111 Coupe. When I cruise in 5th gear at 3000 rpm and 80 mph, I have that noise in the cabin which ends up driving me crasy on long distances. I have to go fater, like 85 mph, or slower, like 75 mph, for the noise to go away. Also the noise at 80 mph is much lower if I am at light throttle, and gets louder if I step on it.

- If I downshift to fourth gear and stay at 80mph, the noise is gone;
- If I slow down at 3000 rpm in fourth, the noise is not there either;
- If I go faster or slower in fifth, the noise is gone.

It is really the combination of 5th gear + 3000rpm + 80 mph that creates the noise. Change any part of the equation and the noise is gone.

I also replaced the prop shaft and rear end recently for other reasons. The noise was there before, and still there after, same way. So I think the rear end and prop shaft are not to blame. Engine at 3000 rpm in another gear doesn't make the noise.

Could it be the trans, and more particulatly when it's in fifth gear that would do that noise only at 80 mph? If yes, what could be the reason? Or may be a combination of harmonics at 3000 rpm and rotation of the trans/shaft/rear end at 80 mph that would get into some kind of resonance?

Thanks in advance for the help!
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  #2  
Old 12-19-2011, 12:59 AM
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Resonance is my thinking. What sort of center muffler are you running?
Are left and right exhausts joined together beside the trans?
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  #3  
Old 12-19-2011, 06:00 AM
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I have the stock r107 tri-y and down-pipes with cross over under the trans and then a stock 6.3 exhaust line.

If the exhaust was to blame I guess I would have the noise at 3000 rpm in every gear I guess. Unless waves combine with some other ones at 80 mph?
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Old 12-19-2011, 06:14 AM
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Hmmm, It's a difficult one and i have seen some really strange things happen and the remedies used were just as strange.
I think it was Chrysler who had a weight on an arm off the back of the trans which damped down resonance . Is the exhaust clear of the floor ? under load the pipes may be connecting the floor and sending a noise into the car.
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Old 12-19-2011, 07:23 AM
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Yes, the exhaust is clear of the floor. I may try and improve clearance but I guess the exhaust would act the same in other gears, which is not the case. It really hapens only when I'm in fifth. I'm trying to determine if there iss something wrong with the fifth (though I don't see how pinions would create such a noise pattern) or a combination of harmonics that happen there. That trans is used on all W108/109/104 107s and I never heared of such problem.

Another thing is that I got all the sound deadening out of the car, first because it was crumbling into pieces, and then for the weight. So it may amplify the problem. I can work on sound deadening though I would really prefer sorting the source of the problem.
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Old 12-19-2011, 11:58 AM
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That's a tough one. Sounds definitely like a resonance problem and I would suspect propshaft mounting, but you already changed that. Do you feel resonance in the cabin, or is it sound only? Sound maybe a standing wave in the exhaust. Try going uphill and downhill in 5th and 3000 rpm and see whether that makes a difference (you should have different throttle positions then). Otherwise, the loading of 5th gear at 3000 rpm must be causing some kind of engine and/or transmission resonance that gets transmitted. Have you checked all clearances around engine and transmission?

Good luck,

Bert
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  #7  
Old 12-19-2011, 12:46 PM
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I feel a bit of vibration in the cabin, a bit like wheels that wouldn't be balanced properly. I've experienced prop shaft vibrations before and they were "cyclical", like they would come on for a few seconds, and then disappear for a few seconds. In this case it is constant at 80mph.

Going uphill and pressing on the gas pedal to stay at 80mph definitely makes the noise louder, and it practically disappears downhill.

I'm still to put the car on a lift to check all clearances again, but I'm pretty sure they're all OK as I put some atttention into it when I put the car together.

Thanks for any ideas and help me check. I'm really scratching my head on that one.
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Old 12-19-2011, 01:00 PM
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Sounds to me like an exhaust resonance - is your exhaust standard? You mention a 6.3 bit - I know nothing about SLs...

To check for an exhaust resonance however I guess you'd be able to replicate the effect in other gears. So in third see if you can get the same result - what's it going to be 4000rpm or there abouts?
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Old 12-19-2011, 01:18 PM
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My exhaust line is a standard W109 6.3 one, very similar to the one on the engine donor car.

No, the noise is not replicable in other gears and at 4000rpm (max torque) it feels like the car is about to take off.
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Old 12-19-2011, 04:00 PM
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could it be a wheel bearing , mine roared only at certain speeds?
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Old 12-19-2011, 04:20 PM
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Quote:
Originally Posted by meltedpanda View Post
could it be a wheel bearing , mine roared only at certain speeds?
The noise is similar to what a worn wheel bearing would do, though with a longer period (if you imagine the noise as a sinusoid). But in my case the noise goes away when I downshift in 4th and stay at 80mph. If it was a wheel bearing the noise would stay.
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Old 12-19-2011, 07:35 PM
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Interesting problem. If the sound pulses in volume but stays at he same frequency it is most likely harmonic. If you could determine the frequency you might be able to find the same note on the chassis or driveline by tapping. If the sound doesn't pulse it might be the intake or exhaust. Stupid question, but do you have an air cleaner installed?
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Old 12-19-2011, 08:17 PM
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In other words, the noise remains the same at 80 mph, louder uphill when I step on the accelerator to keep the car at that speed, and not as loud on flats and it nearly disappears down hill. But it's the same noise, a bit as if I was playing with the volume button on a radio.

Yes, I have a stock air filter.

When I compare it to a worn wheel bearing, I should have said a damaged one as worn bearings make a noise more like a constant rooooooooo while a damaged one makes a noise at each wheel rotation like who-who-who-who

Not sure I'm very clear. I also believe it's harmonic, like the combination of two sound waves, one produced by the engine at 3000 rpm, the other one by the trans, prop-shaft or rear axle at 80mph.
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Old 12-19-2011, 09:50 PM
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5 gear is unique in that it's a 1:1 drive ratio, so load on parts is a bit different.

Have access to a dyno to see if the noise is present when the car itself isn't moving? It may be a bit scary to do it, but a probe pressed against the various underside parts while everything is wirling about on the dyno should find you a part that singing, if there is one (well, except for the drive shaft).

What happens to the noise on turns and lane changes?

-CTH
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Old 12-19-2011, 10:13 PM
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If you are going uphill, you are putting more strain/power in the system. So if your noise (frequency) is there either way, but worse going uphill and less downhill, then the volume (amplitude) is proportionally to strain/power. Thus, engine power causes changes in volume but not frequency. You modified the transmission. Any chance a bearing (with play) in the transmission could be the culprit? Move your hand along the tunnel and feel whether you sense changes in vibration moving back and forth over the tunnel.

Good luck,

Bert
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