possible belt adjustment?

If you hold the pedal tight does it chatter, or does it just chatter while you are engaging it?

Is this something new, or has it chattered since you got the car?

Have you relined the bands? If so, did it chatter before you relined them?

Several things come to mind. Uneven drum surface or crack in drum. Loose driveplate. Loose Universal joint.

How does it work in reverse?

Something to try. Put one quart of automatic transmission oil with the crankcase oil. Sometimes helps.
Norm

chatters with foot to floor, reverse is fine,high speed is good

Try adjusting the low band a little tighter. Sounds to me like it is slipping. It should bottom out about 1" to 1 1/2" above the floorboard. After adjusting put it in neutral on a level surface and turn the crank. It should not turn harder nor creep forward when you turn the crank in neutral, If it does, the pedal might be bent.
Norm

if i ease off gas it stops chattering

If it stops chattering when you ease off the gas it indicates slipping. Either you are not pushing down hard enough or the band needs adjusting. See my post last year on the "Big Bang" There are several things that can cause slippage other than adjustment. If something is interfering with the operation of the pedal it could keep it from applying the pressure to compress the band. In my case, there was interference between the low pedal and the brake pedal. There was also a bent low pedal from pushing too hard. You will see in my post what I did to fix the problem. Your profile doesn't say where you are located. If you are somewhere near San Diego I could come over to help you.
Norm

Could be a missing thrust washer in the rear-end.

Adjust the band and put a 1/2 quart of transmission fluid in the oil. Could also be a worn pedal cam, not allowing the band to tighten up all the way.

My T just started to chatter in low also. But it always chatters (loudly) in reverse. The only other T I have seen back up also chattered loudly too so I thought that was normal. What is the best type of ATF to use? As I recall there are about 4 types of AFT out there.

Dumping ATF in the oil is not going to fix anything. It might not hurt anything, but it is not going to affect the chattering problem any more than carrying a rabbits foot key chain would.