I hear and feel a grinding noise when accelerating. What could it be?

I have a sicon FRS, and I hear a grinning
noise under the front end of the car. I also
feel the grinding vibrations through my gas
paddle. The only time I hear and feel the
grinding is when accelerating, there have
been a few times where I have heard it
when dropping gears when in trip tronic
mode. The grinding does not occur when
idling. The car takes off great! Gears switch
perfectly! The car preforms perfectly, in
other words the grinding is not affecting the
performance of the car at all. I have done a
few modifications to the car, to be specific I
have a cat back exhaust, uel cat delete
headers and a short ram cold air intake.