I'll betcha money it isn't mounts, a dipstick tube or a mechanical issue.
The W body has much less room in many places than the H body. If you haven't already done so, flip your sway bar over. When you put the car in reverse and throttle it a little, the dogbones do allow a little movement. This movement will typically allow a little rub between the swaybar and exhaust.
I'm running a 3" DP on my Regal with poly mounts, a flipped sway and I can still get a little rubbin from it if the proper throttle is applied.
Flipping the sway allows for extra room and typically corrects the issue.