904 Downshift Squall or Howl

With no change in pitch or volume,Ima thinking it might not be in the gear-train, cuz every thing in there is moving at ever-changing speeds.
Unless it was hydraulics related.But I cannot imagine a scenario like that either, cuz at closed throttle and decelerating, the Throttle pressure is at a minimum,while both the Governor pressure and the Line pressure are dropping, so whatever noise they might be making(under unknown reasons) I would expect a change in pitch and/or volume.

So I'll mention this in passing; If you are running a proportioning valve, the rear hydraulic pressure will be much reduced, and the wheel cylinder pistons will not be extending very far. If the shoes were to be in just the right place to excite the drum, but not actually be doing much braking, those rear drums could be singing This singing might not change much in pitch, but it would most likely change in volume..Just a thought.