This just started yesterday. I changed he cabin air filters. No idea what it would be.

Noise only happens when the recirculate button is pressed (on) and fan is on.


2001 Tahoe

 

my first thought is the door to close off fresh air. it may be stuck and the motor maybe trying to close it and the gears are clicking. but not really sure why it would increase with rpms unless it was getting more power but that just doesnt make sense. i would take a 3/4" heater hose a few feet in lengths and put one end up to my ear and the other move around in the engine bay to try and isolate where the sound is coming from. afaik the only thing that happens on the chevys the the flap door for fresh air get closed when you put the ac on max