I have a 79 300TD wagon, automatic trans (there is no vacuum modulator in this model)

I'm getting a sound that is like the sound of "air" being released from a pressure valve. It is located, as best I can tell, somewhere under the passenger front side of the transmission. As the RPMs increase, so the sound. Right before the shift, it sounds as if the "air" is "pinched shut". Sometimes after the shift (usually from 2nd to 3rd) There's little bursts of "air", opening and pinching shut. What I am describing is more prevalent when there is weight in the car, like 3-4 people, and the car has warmed up. It does NOT happen if I am in park or neutral and revving the RPMs, or if I am going downhill in low gear. Now in low gear on level ground and revving up, I get the RPMs up, and of course it cant shift, but the sound occurs and still "pinches out" after a few seconds.

What can it be? Any ideas?
Mike