On my '95 LT1 I can't seem to find anything loose under the car, and all the LCA and TA are tight. But when accellerating moderately, when the 1-2 shift happens, there a loud thump under the car about a split second afterwards.
It's almost like the drivetrain unloads at the top of 1st, then when it shifts to 2nd it loads up again, like shifting from Neutral to drive with the pedal down.
*
I have Spohn LCAs and the stock TA. I did have a Spohn TA that terminated at the tunnel brace, but thats when the thump began, it was so annoying that I removed the Spohn TA and went back with the stocker...now after several months, the thump has returned with the stocker torque arm on there. It does not thump if I accelerate lightly, and the harder I accelerate, the louder/harder the thump is.
I like a hammer hitting the bottom of the car.

Any ideas what I could look at next?
Thanks

 

Transmission mount.

Replacing it is easy, just support the transmission with a jack and change it out.

 

Check out the transmission mount, it is probably ripped/broken. You will probably have to remove it to find out whether or not it is broken. Visual inspection usually reveals nothing. Good time to replace with Poly mount.

 

another vote for tranny mount

 

I guess that could be. When I had the Spohn TA on there I swapped out the LT1 mount for a new LS1 mount and that didn't fix it and the LS1 mount seems to allow the tranny to bounce around even more than the LT1 just by pushing/pulling by hand...so I put the LT1 back on when I put the stock TA back on there...

Guess I'll try a poly one now.

 

me 2

 

I agree. I have broke 3 of them on my car.

 

About to replace my 2nd one now 99.9999% sure its your tranny mount. It'll happen more frequently going up hills and if you accellerate hard off the line.

 

And for the hell of it..... I'll say tranny mount, same thing used to happen on a Fox body stang I test drove.

 

Yep, it was the tranny mount, completely separated in two. I order a new poly one from Thunder Racing.
Thanks to all who replied!

 

Just a side note, some people have said that poly ones on A4's cause some ugly vibes in the car... Seems to not be that big of an issue with M4's though

 

Yep i can vouch for this. When my car was an A4, the poly mount rattled my teeth. When i did the a4 to m6 swap, the same poly mount was perfect.

 

Yeah, I heard that as well and I've also heard folks say they didn't notice much difference. Guess it all depends on how much noise and grinding your drive train and rear end makes. Couldn't be any worst than the Torque Arm mounted to the tunnel brace noises.

 

Yup tranny mount I broke mine and it did the same thing.

 

Here is what my broken mount looks like, took it out today