I installed my GTA Axle-backs this past weekend... Had to take off the Rubber Mounting brackets on both sides to get the GT/CS Exhaust off... so put on new GTA with the Rubber Mounting brackets in place... tightened till could't anymore...and now I am hearing a Rattle coming from the Driverside Exhaust (sounds like metal against metal), sound is most prominant when accelerating..... any suggestions???

 

You may have it to far back and the hanging rod is rubbing??

 

+1. You might also want to check the axle-back/over-axle pipe clamp joint again. Get under car (after cool-down) and grab this connection... push up.. if you get movement you probably need to loosen axle-back clamp.. loosen mid-pipe clamp... re-center tips... tighten mid-pipe clamps.. then tighten over-axle clamps. This will take the pre-load off the pipes and allow for a true alignment and tightening. When I installed my Flows, I had a consistent vibration/rattle at 1300-1500 rpms. After performing above procedure, and torq'n down those axle-back clamps.. no rattle since.

 

Exactly..this should cure your problem..do it to both to make sure

 

And double check the upper two bolts for the mounts above the muffler(the two worst PITA ones on the forward bracket above the muffler). If you tighten the side bolt first it takes a ton of cranking to get the other one tight. I thought I was done and it rattled, creeped in and looked underneath and sure enough it wasn't flush, tiny gap and it rattled. Had to loosen the horizontal bolt so I could get it flush then re-tightened. Intuition has you tightening the the side bolt first. Only rattled at very low rpm like creeping off a speed bump, wouldn't rattle if you shook the pipe.

If they aren't centered, loosening the midpipe and rotating is a snap, just have a helper ready to snug a bolt while you tweak and peek as needed.

 

Will look into these suggestions when I get the chance to get to a lift to look under there.
Kevin are you mentioning the front rubber mounting bracket that is above the Axle-back???

 

I had a rattle around 1250 RPM after I installed my GTAs...turns out it was the exhaust tip contacting the rear bumper cover under load.

At the advice of a fellow forum-goer, I shimmed the rearmost attachment point for the hanger using washers and the rattle is gone!
Tips are still centered in the cutouts, too!