Hey everyone, I've been trying to diagnose this noise/vibration for the last 2 months with my car on stands. Its a 96 Caprice with a T56 out of a 97 Z28. I know there are a lot more knowledgeable people on this board when it comes to T56's so I chose to post this here. I'll start with explaining how this noise started. I brought my car to get 4.10's installed and on the drive home 6th gear had a slight rumble feeling in it. I never noticed this rumble with the stock 3.08s but never used 6th for more than a minute. After the 2nd heat cycle of the gears I was driving down the highway and 6th gear started making noise. Put the car in 5th and it was quiet for a minute. Then heard whistling in 5th, then got stuck in 4th gear. After breaking out of 4th 6th gear had a very loud metallic grinding noise under load. Coasting it was silent. After all this i pulled over at a rest stop. I decided to continue since a tow would have cost over $200. The rest of the drive home all gears were noisy except 1st and reverse. I originally thought for sure it was the rear end since that was just rebuilt. I took the driveshaft off and ran it for a moment and the noise was quieter, but still there. I tried replacing the transmission with another T56, noise is the same. Put the stock driveshaft back in to make sure it wasnt sometheing wrong with the Denny's I had in there. Same noise. I just took the tranny out again and took the Street Twin apart it looks fine. The pilot bearing looks and feels good too. I've looked over everything and still dont know what to replace next. The only part im not 100% sure about is the throwout bearing. I've heard its a very common problem with these. It feels smooth as glass when I pull the bearing out towards the rear of the car and only the inner bearing spins. If i spin it without pulling and the whole throwout bearing spins as a unit it sounds metallic. Is this normal or possibly the cause of my noise/vibration?

Summary: Loud grinding metallic noise with a slight vibration. This noise/vibration is almost only heard under load. Coasting is basically silent.
-Silent in nuetral, and silent in gear with clutch in.
-1st and reverse are nearly silent but under higher loads can make slight noise.
-Clutch engagement and disengagement seem normal.
-Tried using a different T56 with the same results.
-Street twin clutch taken apart with no obvious problems.
-Pilot bearing looks and feels smooth
-Throwout bearing I'm not sure if it should sound slightly metallic when spinning the entire bearing as a whole unit. Smooth as glass when spinning only inner part of TO bearing.

Sorry this is so long but I thought the more detail I gave the better chance someone would have in diagnosing this problem. Thank you for taking the time to read this.

 

It would probably be a good idea to take off your diff. cover and look around. Since you've swapped trannies and driveshafts it pretty much narrows it down to the rear axle only.

 

I forgot to mention that was the first thing I did when I got the car home. I took the cover off to look around the diff. Everything looked ok and the bearing caps were tight. I also forgot to mention I listened under the car with it running in 3rd gear. The noise sounded to be clearly coming from the clutch area. This is what also led me to the throwout bearing. Only other idea I had was the noise was travelling from the rear end through the driveshaft and coming out the front of the tranny. I didn't think this was likely so I'm still thinking something up front. Thanks for the reply.