Been wondering for a while...

[99'Yukon]

Having a hard time figuring this one out. Not every time, but very often I have what sounds like a cd syncing sound coming from behind the dash. I know this sounds weird and it is, but that's the only way I can describe the way it sounds lol. My only guess would maybe be the blower motor resistor contacts/coils being corroded or boogered up, making it where it doesn't know when to switch over on the 3 knobs on the ac/heater control panel. A good example would be; say I go to turn the middle over from cool to hot, and then change it from floor up to coming out of the dash vents, it starts to make the syncing sound and then you hear the blend door kick in. Very strange noise that I've never really heard on any other application and it just has me stumped. Also my tiny vent on the bottom of the steering column has never had anything blow out of it. What else could cause that? I appreciate it in advanced guys!