Air pressure release flap

Hi, this may be a stupid question but here goes.

Have had the new car for a couple of weeks now, and it is great.

However, has anyone noticed that when you close the last door you can hear a sort of opening flap sound somewhere near the boot on the offside, which then sound like it flaps back again.

I am guessing that there is a flap somewhere to release the internal air pressure which would otherwise make the last door difficult to close.

I have never really thought about it before, but I guess most cars must have something similar since all the vents auto-close on cars with climate control.