has to have a pump on it, usually located where the torque converter input is. Probably a bad seal causing it to pull air.
transmission will have to be separated from the engine and the torque converter removed to get to it to change out the seal.

Also in the troubleshooting manual for the transmission there is a section about bubbles in the fill tube.
it says:
- Air leak on the suction side of the oil pump
- Oil level not correct (over full)
- Oil too hot
- Problem in the torque converter

so any side on the suction side of the system can be pulling in air causing the bubbles or being over full or getting too hot.