I have a 99 blazer LS 4x4, anytime I have the heat on 1-2 the fans makes a noise like it's rubbing against something when they're turning and I can only hear it when its down that low, but not so much on 3-4. Don't know a real lot about trucks so I hope you understand what I'm saying. I had an old dakota that did the same when they were turned down..I was wondering what makes that noise..and if theres anything I can do to make it go away?

 

Possibly a failing blower motor. Or, something in the duct work. Replace the motor, a lifetime warranty motor with the squirrel cage on it was only 25 bucks for me......