at about 45-55,got a vibration, noticable in 5th.
put into neutral rev engine , no vibs from engine.

could it be a diff mount,, or Ujoint,, or???

 

Check your driveline for any play, U joints and the 4 blots that mate it to the rear diff.

 

U-joints usually whine, they eventually clunk when worse. CV shafts will vibrate under acceleration (Plunger joint), or will click when turning (outer joint). Tires will vibrate at certain speeds only and will eventually go away because the tire is moving so fast it will cancel out the vibration. It is noticeable at low speeds, and may peak at a certain speed like 100 km/h or 60 mph and then just disappear as you accelerate.

It may also be a clutch condition. Is it as you clutch in? Or as you clutch out?

 

Could be the driveshaft lost a balance weight (or needs one).

 

More often the driveshaft gets dented than loses a weight. Its entirely possible though.

 

well gives me somthin to do, jackit up and start lookin for a prob.

what way to check for diff mount?

 

what way to check for bad diff mount, got vibes.

 

Id check the rear diff mounts. Especially the front one by the driveshaft. But i think that would only make a clunkin noise on take off or decel. Id check check the driveshaft for play first.

 

Almost certainly a rear u-joint...and you can't tell until you unbolt the driveshaft. Ask me how I know this.

 

I agree with the shaolin. I've had that problem on other vehicles and now my 86 RX-7 FC. does it go whooom whooom whooom? deep harmonic bass sound? U-joints. Replace them both you're already there. it's hard to tell until you get them off. That's your problem.

 

S5 TII purchased new. A few years ago I jacked up the rear and took the wheels/tires to the dealer to have new tires installed. The car was lifted in the rear for maybe 4 hours. I installed the wheels/tires and took it for a ride; there was a vibration in the rear, but not before. Thought it was a bad balance. Took them back and the balance was spot on, so I checked all the rear suspension components, d/s, trans mounts etc. Drove me crazy as the vibration started around 45/50 mph. Long story short, it was a bad right front motor mount. Evidently with the weight forward, it was enough to split the mount and make the engine slightly askew. Replaced the motor mount and all has been well ever since.

 

ya know after thinkin about it,, the vibs started after i made a tow dolly run for around 200miles.

tow dolly fronts up on dolly,, rears down on road? maybe somthin happened to mounts!