I have been having this problem for some time now. It was happening even before I installed my new clutch so that is not the problem.

When I take off from a stop and do not give it enough gas, I feel a violent slapping noise on the floorboard. It feels like it is near my my legs around the trans tunnel somewhere, can't really pinpoint it. Does anyone have suggestions on what to check, or what this could be?

I have recently changed the trans mount to a poly mount so that should not be the problem, it was poly before but just worn...

I have a spec stage 3 clutch but this has been happening before I put that in.

Could it be the headers slapping under the tunnel or what would bad motor mounts sound like or symptoms be?

I dont know, I havent been able to pinpoint this forever...

 

I had a similar problem with some headers in my old 3rd gen...the flanges were right up against the floorboard, and would vibrate against it during certain RPM. Make note of when/how it happens, then put the car up on ramps and have a friend rev it while you look (car in park and E-brake on). You should be able to spot it from outside the car. HTH!!!

 

Check your torque arm mount where it mounts to the tranny.