2020HDBLACKbeast

I bought my new 2020 Silverado 2500 HD about 3 months ago and every time you open the door after it sits for a little while I hear this whoosh of air coming from the rear end that almost sounds like air bags decompressing. I’ve checked the truck has no air bags, I called my dealership they have no clue what I am even talking about. It’s not causing any issues that I’m aware of. Can anyone enlighten me as to what this could be? Thanks in advance