It sounds like a heat riser rattle I once had in my 65 327/300. With your foot on the gas you're forcing it open , but as you back off it closes and rattles similar to a truck pipe flapper. Put a glove on and reach down at idle and try opening it.

The heat riser was the first thing we thought of a while back. The spring on the riser seems tight but we have not taken it apart yet. (IT IS HOT DOWN THERE). Again, today we had a beautiful clear day here in the Chicago area, so I took the car out for a spin. Yes, it seems that the noise is most apparent when I let off the gas (when there is NO pressure on the accelerator). Even the slightest pressure on the gas quiets it down. However, at about 3500 rpm (about 75 mph), the sound is almost like a flapping. It is pretty bad until I go back down to 3200 rpm or lower. Then with the accelerator steady, no rattle. Seems this might be an easier place to start than the transmission! Thanks. Any other thoughts are greatly appreciated.