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Discussion Starter #1
Got to ask for help from those with knowledge again. My S4 (new to me) has a rumble/deep rattle/minor shaking when accelerating up through 2nd and third and I cant figure it out. Engine mounts are new, u-joints are ok and when taken to the local expert he says "they all do that". I dont believe this a bit, something is not right but I cant figure it out. Its only when driving, not when reving engine. Its noticible and the best way I can describe it is I once had a sport exhaust on a Spitfire that did a similar thing due to exact harmonic vibrations between this particular exhaust and the Spitfire. Eventually replace the exhaust.

My S4 has an original exhaust system so I assume its tuned to the engine.

Anybody else have ideas or thoughts on what it could be. It goes away once you are in thrid accelerating, it can be felt through whole car, its only accelerating 1st-2nd, or 2nd to 3rd and not when cruising or decelerating....

Anybody else have this on their car?
Many thanks
Britt
 

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Our '84 doesn't do that.

How is the transmission mount? Even new they seem pretty sloppy to me. And the hole for the mounting bolt is off-set. Could it have been installed upside down?

Will it make the noise if you rev the engine while sitting still? If so, get it up on a lift or sturdy ramps and get underneath to locate the source.

If it will only make the noise accelerating in gear then I'd have to suspect something in the drive train -or- something is shifting when the engine moves under load. If you suspect something in the exhaust is the source of the noise due to a pipe or muffler hitting the body, try using a piece of wire to temporarily pull the exhaust system to one side or the other. If that moves it away from whatever it's hitting (and thus the noise goes away) then you'll need to figure out how to re-adjust the position of the exhaust system. Sometimes a 1/2" is all it needs.
 

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I agree that low frequency banging/vibration is most likely coming from the center support. The rubber surround gets weak as it ages and allows the center of the shaft to move around too much. It is not difficult to replace if you have access to a 10 ton press.
Another possibility is a tight U-joint.
A loose U-joint will usually cause vibrations at higher speeds.

Ed Prytherch
79 Spider
88 Verde
 

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Discussion Starter #6
Several good ideas guys so thank you. No, it doesnt happen when sitting and reving the engine so testing it on a lift whiile under load and driving through the gears is going to be tricky. I will check the central bearing however it is only when accelerating in the lower gears up and not when in 4th or 5th and just putting your pedal down. Its not the clutch or transmission internals, could be the drive shaft supports though!
 

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The last time that I had a bad center support it was at its worst when I made uphill starts. The bearing itself may be OK. The bearings seem to outlast the rubber surrounds. But it makes no sense to put a used bearing into the car after you have invested the time in removing and re-installing the driveshaft.
Ed
 

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Most likely

"it is only when accelerating in the lower gears up and not when in 4th or 5th and just putting your pedal down."
Those are the exact symptoms of a dodgy Center support.
One can never be sure with someone else's car on the other side of the world, but check #2 in the list, the flexible coupling and replace if it feels soft in spots or shows signs of cracking or oil degradation, and replace #9 center support.
Oddly enough, when my center support was dodgy, the one I took out didn't feel that different in the hand to the new one I put in. I guess the design emphasis with these cars was on eloquence not robustness. but you got to sort it.
There's nothing much better than putting your foot into these cars and getting smooth strong response combined with their handling.
Also, if your into this car, get yourself a Cardisk repair manual on cd.
Thats the cheapest most practical way to have all the info you need when little things pop up.
Happy motoring.
 

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another big culprit is the exhaust system , as remedied the Spitfire's issue.

your new S4 is pretty unconventional in its exhaust path. check it out. it snakes up and down and around and comes very close to the floorpan and even gas-tank. only under real torque, will it vibrate and when its tapping against something, it can cause confusing sounds that seem to come from a strange place under the car.
 

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Discussion Starter #11
My Spider has only 31,000 miles on her. Wouldn't that be a little soon for a driveline to wear? What miles did you have when yours was worn out?
 

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It is the rubber surround that goes bad. Rubber ages independently of miles driven. Ever driven on old tires with lots of tread?
Ed
 
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