thumping when I floor it

There are 100,000 miles on my newly-purchased WS6. When I hit the throttle hard from a standing stop or lower speeds (up to 35mph), I get this real loud thumping. It feels like something is hitting the floorboard, and I think it's coming from the right rear (but I'm not sure about the spot at all). It doesn't stop until I let off the gas, and it quits immediately. I thought at first it was wheel hop, but now I'm not so sure.

This has happened on my Ford T-bird SC before; in that case, it was the rear axle hitting the body. The rubber pads that are supposed to absorb the impact were worn out. Ford called them 'insulators'. I've been under the Firebird, but I can't find anything like that. I almost want to say the transmission's vibrating and hitting the floorboard.

Has anyone had this problem? Can you tell me what to look for, or what's causing this noise, and metal-to-floorboard thump?

Thank you.

After some surfing, I found several descriptions of this occurence that were posted by other readers on other forums. It seems this is a common affliction plaguing the 4th Generation 'Birds, and is fixed by replacing the transmission mount.

I was fooled by the appearance of my mount when I looked at it because I couldn't see any damage, but the rubber insulation had sheared from the metal bracket, allowing the tranny to float unsecured to the crossmember. Once you purchase a new tranny mount, you'll know exactly what it looks like and can find it under your transmission. The tranny "rests" on it, which rests on a crossmember, just before the driveshaft's front linkage. Put the car on jacks, use a jackstand with a couple of 9" long 2x4's to lift up the rear of the tranny, and R&R the old one after unbolting it. $30 and 15 minutes tops.

I needed new tranmission mounts when this happend

my third gen did the same exact thing. replaced the tranny mount, and now all is well.

Anyone have a good source and part number for the tranny mount?

jegs the will have a mount made of a stronger material

sounds to me like its your tranny mount....i had mine go on me also. I read stories of people going through them really quick, and the plans i got in mind for my firebird i know i could have gone through a couple of the stock ones easy, so i spent the extra money and bought a aftermarket one that had metal hooks in the rubber so that when it tries to pull apart from the sudden torque the metal hooks engage and keep it from comming apart. I got mine from jegs and it costed like $50ish?(i think) that might seem kinda high for a tranny mount but if you are going to drive it hard or race it, you are actually saving in the long run.....the factory mounts may be less, but if you go through 2 of them you already spent enough to buy this one that wont go out on you. here is a link to the one i bought
http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=3484&prmenbr=361 not saying you should go with this one necesarily, just dont go with the standard oem one....

You can buy an Anchor Trans-mount from Kragen/Autozone for $25, w/ 1 year guarn....It takes about 15-min, a jack and removal of 8-bolts to change...its right at the rear of your transmission...your driveline balancer is hitting your floorboards under your center console...
Mine broke after I put a B&M shift kit in my firebird...

http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=880&prmenbr=361

This is the energy suspension poly mount...

355-3-1108G

355-3-1108R

You can use either of those two, they are the same but different colors. I just replaced mine. Make sure to use a little loctite when putting it in. You will have a little more tranny vibration too but it won't snap again.