'97 740il - Noises from HVAC system

There's a thumping/pulsing type of background noise that seems to come from the HVAC system - noise goes away when climate control is turned off.

Does anyone have any clues or experience as to what may be causing this?

Thank you.

Ned

Have you found the source of this noise yet? Mine makes a similar noise, it kind of sounds like there is water in the dash.