If it is your AYC, I'd treat it very seriously. A new replacement
diff is circa £3k.
Change your AYC fluid, and have the hydraulics bled. If the noise goes away and then returns, try disconnecting the AYC fusible link,the diff should then act as a normal lsd ( AYC fluid still needs to be changed though).
If the disconnecting procedure makes the noise go away , you may be part of the dreaded AYC recall, which affects some 4's and 5's GSRs with AYC diffs.
Read all AYC threads on this forum and Coordsport forum (under transmission).
Check your chassis number and send it in to the Colt Car Company to find out if it is part of the recall, they may be able to get you a correctly programmed AYC ECU, but some other people havent had much luck.
If your diff does go bang, better to replace it with a normal lsd as per the RS model , the cost of this is significantly less. I believe the Cusco
replacement is around £1k, still not good but better than 3k.
The maintainence of the rs diff is less frequent also at 24,000 mile drainage intervals.
-Mr Mime