Bam! Bam! What was that?

I've put some minor aftermarket performance mods on my '97 Firebird Formula, and my biggest problem to solve at the track has seemed to be getting some better tires so I can "hook-up". Right now I'm turning
in 13.9 @98mph, but badly spinning my rear tires off the line (I know...I shoulda gotten Drag Radials or Z rated tires, at least).
Anyway, I recently bought a Hypertech programmer and have subsequently developed a new problem I can't explain. If I jump on the throttle too quickly I hear a BAM! or sometimes BAM! BAM! coming from beneath or behind the drivers seat. I can feel the shockwave go throught the car. I can get it also when the car is moving and I open the throttle to downshift, but it's most pronounced off the line. I have an SLP Shift Improvement module that I've put on a toggle switch. With that off and the Hypertech shift improvement off, I still get the BAM!
Thinking it might be the weight shift I bounced the rear of the car in my garage, but didnt' hear anything.
Next I'm thinking of marking the tire position on the rim to see if the tires is slipping on the rim.
I'm concerned it could be some slippage in the tranny or differential.
Any thoughts?
Thanks, Zack ><>
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but badly spinning my rear tires off the line (I

Easy enough..and common...check your tranny mount...
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My IROC made a sound very much like that.... Check the rear transmission mounts my guess is you need new ones.
Gary
but badly spinning my rear tires off the line (I

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