Close mic'd recording of constant air movement through a door gap. Take 1 of 3. Across the three takes I made slight adjustments to the position of the door and mics to alter the pitch of the tone produced, and the amount of "rippling" or modulation. Some slight vent rattling is also audible.
@snortingcarrots Yes, there is a little low level rattle in there from some vibrating ventilation ducting. Such is the nature of field recording. It can be reduced slightly by cutting a few dB around 3.5kHz.
I don't know if you've noticed but there is a high pitched beep or peep sound in your recording. I'd love to use your sounds (as they're awesome), but what I do amplifies that tiny imperfection, so I can't really use it. I love working with ambient "foley" sounds and whatnot, so I find your recordings really useful, if not for the little beep thing.