I have the same problem with my 2001 Cavalier.
GM's soloution to it was to change the oil in the transmission to a heavier viscosity and then to advise me to "change gears at a diffrent RPM". My car was doing it so loud you could hear it bouncing off the houses back to me if the windows were open in the summer.
This was, word for word, GM's advisory to its service depots as of 8/1/01:
"There is no correction for the rattle noise. advise the customer to raise the engine speed between gearshift, this will reduce the rattle noise level".
This was, again I must stress, after they changed the oil.
I was told by someone independantly that it sounds 'a lot like a bearing dancing around on something''.
GM put me off untill the warranty expired without solving the problem.
Sell it dude.