AC Odor

When I turn off the AC in my 1999 Sonoma and switch over to the outside air vent, an odd smell comes from the vents. It does this even if I drive with AC, park for a while, then switch over to vents immediately after turning the engine on. The smell only lasts for a minute or two. It's kind off a musty-humid odor. Is this normal/common or is there something growing in the AC box?

Vehicle is 1999 Sonoma, 4.3L, Auto.

Thanks.

Mine's smelled like that the whole time I've owned it...

I have that too. I was afraid it was refridgerant or something. It's a musty iron smell for me. I heard the refridgerent is odorless anyways.

The smell is mold.Go to your GM dealer,they sell a spray can,that when sprayed into the heater/ac box takes away the odor and gets rid of the mold.For how long,I don't know.