I have a '92 SSEI, and if I rev the engine while in park, I get a loud clunk that sounds like someone hit the engine with a hammer. After some research, I was told that GM had some problems with the overrunning sprag in some models, and it sometimes catches when in park. The car actually tries to move a bit when this happens.

I was also told, JUST DON'T DO IT. Apparently it can trash the tranny if you keep doing this.

So, I don't rev the engine, and no harm is done !!!!!!