Noise in gem car drive train

2002 Gem car, when I take my foot off of the accelerator pedal there is like a rattle sound. Maybe like a gear rattle.can someone tell me what may cause that

That is caused by a worn Input shaft. Lots of info on this site. But in short when the motor and input shaft (on the transmission) are under load things are quiet. When you let off and the gears are not under load you hear/feel a loud clanking/vibration.

Replace the input shaft with a upgraded one and bearing and oil seal and you will be good to go. Rodney (Old Housboater) here sells kits

could also be the rubber bumper which goes into the splined hole on the bottom. Does it also go clunk sometimes when you start moving?

No clunking just a rattle when letting off the accelerator pedal.
Just order houseboats kit