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I'm going crazy chasing down a vibration on my '02 SS convertible A4. I've owned since new. I have a host of mods - mostly bolt-ons... headers, Y pipe, Magnaflow, SLP's Eibach/Bilstein package, LCAs, SFCs, yada, yada... I've had all this for some time and have had no issues. A few months back I begun to get a bad vibration when at highway speed. It only lasts a few miles and then it's gone. Around the neighborhood... no problem, but once one highway (i'm only approx 5 mins from the highway) and go up to speed it sounds/feels like something is going to fall off. Only happens in gear. Accelerating in neutral, no problem, which leads me to think driveline. Again, after warmed up it goes away and remains gone for the rest of the day.
Had my shop go over everything with a fine tooth comb. Saw some binding on the rear u joint, so it was replaced. Didn't fix it. Rebalanced the tires, which proved difficult so being they were 9yrs old, replaced them. Didn't solve the problem. Inspected the rear diff for any problems (had a carrier bearing fail a year or so ago, so it was rebuilt), no issues found. Checking suspension, wheel bearings and pretty much everything else we could think of, no problems found. I even replaced the EBC disks which were getting on in age with Delco blanks. Still have the vibration. The car has 55K on it, mostly only goes out on the weekend.
Anyone want to take a guess??? Running out of things to check.
That I've not done yet. Looked up what TC shudder would be like on YouTube and it's pretty much like that. Will check it out. Thanks for the suggestion!
If it does seem like it's the transmission ...
How the transmission fluid look?
Has the fluid been changed recently?
Generally, I'm not a fan of additives but Lube Guard Red can help with transmission shutter. Local transmission shop recommended it at transmission fluid & filter change. My 91 RS had a 700R4 and eventually it developed a little shutter on the 1-2 shift. Added Red Lube Guard at the extra transmission & filter change. The shutter went away. The 700R4 had 389,500 miles on it with no issues and still shifted well and didn't slip when pulled for the LS swap. No rebuilds on the 700R4 or other repairs.
Of course do some research before taking my recommendation
When I had that symptom.....kind of come and go vibration...
It was worn Torque Arm bushings. Messed slightly with the pinion angle. Put a new factory clam shell bushing on and it was gone for good. And the bad bushing that was 15 years old looked just fine. It looked bad after we took it off. We just guessed right and put a new one on, problem solved.
Follow up that I think we found the problem. I have Kooks custom headers with their Y pipe and cats. It runs under an X brace under the car (I have a convertible) that had to be modified to fit over the pipes. It's about right where the Y meets the exhaust. I've had it for years now with no issues. Now looks like when cold the exhaust is grounding on the X brace causing the vibration. Adjustment made. We'll see what happens. I'm running factory trans and engine mounts, so thinking after 16 years they're starting to sag just slightly enough to touch when cold. More to come.
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Chasing down a vibration - suggestions?? 
