Knocking from under Carriage VR6 Jetta

Ok, here I go again, I have a 95 VR6 GLX Jetta, just had the header pipe (Front exhaust pipe according to Bentley Book) replace. Also replaced all the Motor mounts (transmission Mounts, if you read the Bentley Manual) The car runs great and quiet, that is until the engine warms up, it then at times not always, as I am upshifting and stepping on the gas there is a knocking comming from under the car just below the rear seat. I sometimes feel it on gear shifter. As I mentioned before, this only happens when the car is warm or after 15 minutes of highway driving (I do alot of that here in Houston) also, the knocking only happens as I am stepping on the gas, when I ease of it or get to my cruise speed the knocking stops.
I have my clues as to what it might be but would like to hear what you guys have to say about this. any imput will be greatly appreciated.
Tacuvito.

It's probably the exhaust hitting the body back in front of the rear axle. It probably got shifted when they replaced the header pipe or the rubber "donuts" were broken. Should be fairly easy for them to fix.

true dat!