Trans occasionally thunks

[Nixon1]

I know what the instant reply is going to be...check trans mounts. Yes, I beat the hell out of the car. But if I nail the gas from a stop, there is no thump or jerk. This thunk is different. I can't physically feel anything, but I hear a dull metallic whack when I'm idling and step on the gas to go sometimes. Feels like something INSIDE the transmission is thunking. The trans has over 150,000 miles on it and slips badly. It's loaded with 2 bottles of Lucas Slipfix, the line pressure is bumped very high, and the fluid has a tendency to aerate, which I suspect is from being overfilled.

Could the sound I'm hearing actually be coming from INSIDE the transmission because it's so totally shot? I know I can hear it thunk when it grabs overdrive solidly (on the throttle or completely off the throttle). Cruising, it bounces into OD. That is, it engages, slips, and then reengages every time.