A little background info... I currently have Berk HFCs and a Fast Intentions cat back exhaust that had to be modified a bit for fitment (the midpipe muffler was hitting my drive shaft). The exhaust shop cut a few inches off of the Y pipe (which displaced the muffler toward the front of the car) and added extensions to the rear portion of the cat back (to make the tips sit flush with the rear bumper). All of the welding looks proper as far as I can tell.

On to the problem... After my car gets a little warm and the idle drops to around 700-900 rpm, there is a really bad loud grinding/rattling/screetching noise coming from the rear left exhaust pipe. If I put my foot onto the tip of the exhaust, the noise goes away. If I let it go, it returns. The noise only happens at this RPM range. If I accelerate from a warm start, there is a brief noise because the rev meter passes through the above mentioned RPM range. Other than that, exhaust performs as normal.

The sound is not coming from the HFCs or mid-sections of the exhaust. I hear a distinct sound from the left rear section of the cat back.

Any ideas of what this could be? I checked to see if the exhaust is hitting the sway bar or any heat shields and it is not in contact with any other part of the car.

Any help is appreciated. Thank you.