UPDATE on 99Tahoe making strange sound

[Treblig69]

I posted a problem with my '99 Tahoe a few days ago. I have narrowed it down for anyone else who might have the same problem. The sound my Tahoe makes is kinda like the sound a rachet would make if you grabbed the rachet where the socket goes and spun it around real fast. It's kind of a muffled clicking that seems to increase with RPM. Today I noticed it only does it when I put the vent selection knob in the windshield vent positions (there are two). The clicking is driving me crazy and I was hoping someone would know what engages when the vent selection knob is put in the windshield positions. This problem started this winter..it figures because that's about the only time I need the windshield defroster!

Treblig in Tex