I have a 98 SS convertible M6

-new OEM zexel/torsen rear
-subframes/LCA relocate brackets/BMR panhard/SS springs and shocks
-TSP rumbler exhaust, K&N FIPK2 intake
-T/A diff cover/stud bolt kit

After the new rear end got put in, it started making a ticking noise coming from below the center of the car, just behind the front seats, sounds like a bicycle when you have a card through the spokes..
-only under load
-only while moving, gets quicker as speed increases
-recently has started changing loudness and sounding more random whereas before it was very steady

The shop that put in the new rear (a trusted buddy) has looked at it many times, even taking apart the rear to check it and cannot find a problem, the rear driveshaft U-joints have been replaced as well.

Has anyone encountered this problem before, or have any idea what it could be? There are no signs of any problem under the car, even now!

Thanks, Adam