2002 Hyundai Accent with automatic transmission. This problem happens only if I floor the gas gas on the highway (trying to pass somebody or get out of the way in a hurry) or if I apply too much gas after coasting around a sharp turn. My car will then downshift with a jerk and an audible THUNK!. Right now I just avoid those situations and it shifts through all its gears fine, it only happens when I cause a sudden downshift. Transmission fluid level and color fine.
Since this is happening under heavy load conditions, have the motor and transmission mounts checked. The clunk is something shifting position under load.