Loud clunk underneath car

Whenever I accelerate from a dead stop or close to dead stop I hear this loud clunking noise coming from the center area where the rear seat is located. It sounds like something is banging the underside of the car. Once I get up to speed (30+ mph.) there's no clunking when I accelerate. This doesn't happen when I rev it while idling or accelerating while in reverse. What could cause this noise?

I actually started having the same problem tonight (July 24). I got on and looked around for a while before I found this. I think me and you are on the same page as far as the problem we're having. Here's the link, hope it helps both of us:

http://web.camaross.com/forums/showthread.php?t=381101