I wish this thread wasn't so old since I have the same problem? hmmmm.??? Anyone ever hear of fbody cars doing this (especially when cold)? Mine sometimes even does it when at operating temp but seems to go away when warmed up (typically) or if rpms climb at all.
My 2000 Z28 does it too. Always has. Owned it since 2003 when i picked it up with only 7k miles on in. Took it in after i bought it and had them listen to it. They said its just the exhaust warming up, nothing to worry about. Only does it after the first few shifts down the road. After a block or two it stops. Almost sounds like stones bouncing around in something metal. Been doing it 7 years and 50k miles now and nothing has fallen off or broken yet. Easier to just not worry about that and worry more about the passenger side window motor which is slowly meeting its demise with each request to rise to the occasion.