Several causes, but all is just tossing darts in the dark.
If you feel comfortable with it, do a flush and filter change. Pull the pan and inspect for worn or burnt clutch material, pieces of aluminum, bronze bushing material, or other debris.
Depending on age of unit, the direct and forward drums rub against each other until the land wears through then the snap ring pops out.
If the unit has the locking converter, this is common when 3 to 2 downshift happens, yet torque converter clutch doesn't disengage cleanly. The band and intermediate clutch pack will do this when it starts wearing down. The other packs will start slipping but you usually don't hear anything.
This happens when you engine brake by manual shift from Drive to 2nd and hammer the brakes at same time to get the tires to chirp or leave a patch, wears out the band.
If a shift kit was put in and 2nd gear oil supply was drilled out and accumulator spring removed, gives very hard snappy up shift from 1st to 2nd when in drive. This can wear out intermediate clutch pack too soon and can cause chatter. Sounds like dinner plates rattling when down shifting.
