Here's a clip from someone with the "off road" version of the GMPP axle back.
http://s614.photobucket.com/albums/t...t=MOV02569.flv
I should have mine later this week and I'll post my thoughts on it. I'm expecting it to be pretty tame, because I have an L99 with all the resonators and cats. But I'm planning on getting long tube headers and high flow cats sometime in the future, and that will make it a lot louder.