when i am at WOT i hear the exhaust change tones. first it sounds like a normal flowmaster tone and then it sounds like i am running with no muffler. when i stop flooring it, the tone goes back to normal. is there something wrong with the welds of the muffler or is there something wrong with the muffler itself? i have had this muffler since i guess the summer of '99. thanks for the help....

 

Look for a leak or a crack...

 

Are the RPMs changing? It sounds like maybe its doing it when your converter locks up?? BUt your converter shouldnt lock under WOT.