I've been chasing down a vibration at 80+ mph for awhile now. It's more like a buzz/hum and not an imbalanced tire. It's speed related and not RPM related. It does it in 3rd-6th gear gear, neutral, on the gas, off the gas, and at a steady speed. I can feel it through the shifter and clutch pedal. The wierd thing is sometimes there is no vibration, and other times it shakes the change in my cupholder. For example last weekend I took a 300 mile trip. For around 200 miles it was vibration free, then I stopped for a snack and gas and the vibration appeared. It only goes from no vibration to vibration or the other way after I stop and park the car. I've tried pushing in the clutch a bunch of times, tried changing gears a ton of times and nothing I do while driving either brings the vibration on or makes it go away.

Things I've checked/tried
-Ballanced driveshaft (3.5" LPE aluminum driveshaft)
-tried LS1 driveshaft that's in a vibration free car
-had a bunch of different wheels/tires on the car
-Just put in new pinion/carrier bearings in the rear
-did it at .007" backlash, I have .010" now since it was .002" after the bearings and I had to re-shim it.
-Tailshaft bushing still feels night, no leaks coming from it
-u-joints in driveshaft feel fine.

I had a loose pinion nut so I pulled the rear end apart and replaced the carrier/pinion bearings with timkens. The old pinion bearings had an irregular wear pattern, it looked like the pinion was wobbling. The part I don't understand is how it can go from being completely smooth to vibrating after parking the car and getting back in it.

 

No one has any input on what this could be? It's driving me crazy on the highway.

 

i had a pretty strong vibe at 100+ mph that turned out to be pinion angle related

just a thought

 

i had something like that too and it turned out to be a worn/broken trans mount. If yours is stock throw a poly in there. Its super cheap and could fix it or help you narrow it down even more. If it is a bad tranny mount it could be the cause of all the wear problems

 

I have had a poly mount in my car since I've owned it. I even tried switching poly mount brands for the hell of it.

 

poly mount is a bad word....

Try the $16.00 94-97 Firebird Transmount from NAPA.

 

I don't feel like replacing broken transmission mounts monthly.

 

So I take it you have the TQ Arm connected to the Transmission still? I just went from a Spohn Body mount to a BMR TQ Arm that connects to a UMI Torque Arm Relocation Kit.

I hated the old setup, I had so many issues, such as vibrations and rear end noise.

Also, I would like to say that my launches are 10 times better, with no wheel hop.

 

Sounds like you've already found a potential source and/or area for your problem. I've got a similar issue w/ my car at the moment (except mine's @ 110+ mph), unfortunately I haven't had the time to explore its' cause.

 

Was getting a really bad vibration over 100mph (on a closed track, of course ). Replaced too much stuff at once for it to be easy to diagnose. Turns out that my MWC T56 tranny cross member was pressed up against the tailshaft housing. Clearanced the tranny and cross member and the vibration problem was much better.

Still some vibration, but it's livable. I attributed it to the torque arm. The MWC street kit only has one tiny little poly bushing with everything else rod-ended so it might as well be a race kit.

So to sum up, all you guys with MWC 9"s and T56s need to check for interference.