Ouch! I had the EXACT same noise in my car. For me i threw a rod, and instead of rebuilding i opted to get a lower KM engine put in my car. Happened right out of the blue as well.
Total bill was $3800 Canadian when all was said and done. Sorry man!
i know its junk, but what about needing a differential? i just need to know if the '97 gtp engine will work in my car before i go out and try to put it in