Kongfish

I have a 2007 Honda Accord SE V6
The past few days when the car has sat for longer than several hours and I back up I hear a moan coming from the rear area, the braking or not braking doesn't matter, but the moan only happens that one time when doing a cold start back up. I have read where people are saying it is a caliper, warped drum, disc, pad, or clip on pad.
I guess the noise could be one of those but I really don't think that a rub or rotating is what this noise is. This noise isn't like a rub sound.
This sound has a more of a diaphragm sound, I've heard the sound before when
I had a partially collapsing suction hose that controlled a diaphragm.
It just seems as if this sound is a air based noise that never changes in tone, based on distance backed up or braking or not. It's a set 3-5 second sound always the same tone.
Anyone got any solid knowledge on other Hondas that have had this same issue?