What is this Sound?

[Timothy Ferguson]

I drive a 2000 Ford Ranger. And every so often when I turn on the A/C or defroster which starts the A/C anyways I hear a loud thud soon after I hear the A/C compressor click on. Is it just the bleeder doors slapping shut maybe or something else. I've heard in one other Ranger and mine does it every so often usually when cold out. It doesn't seem to effect the performance of anything and I can't tell you when the last time I heard it before thats how long its been. Anyone with ideas. It does sound kinda like a flap or a door closing quickly.

 

[another Todd]

Sounds like a ventilation system door closing. They are vacuum operated (usually) and take a few seconds to complete their moves after you change the settings on the a/c heater controls. Several different doors open or close depending on the setting you select.