Yesterday I found out what the problem was on my 04 Monte. Under the hood,their are two support brackets that run from the fender to close to where the front engine mounts are. They were loose,and they would move back and fourth.Friend of mine would put it in gear,and it would move,creating the noise.Smooth as can be now!! Thanks everyone for your input.

 

I had a clunking sound on my 2000 Monte Carlo and the check engine light came on and the whole car shook when shifting. Turned out to be a bad sensor in the transmission. You can just replace the sensor, you have to drop the engine and drill a hole through the valve body and install a kit. I had mine done for $900 and works like new. This may not be your problem, but sooner or later all monte carlo trans sensors are going to give out. They put a poor sensor on them. The same trans in on Malibu's also.