Fan/Heater/Air Noise under the hood - Please help

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Hello, new guy here. I have a 1999 Silverado 2500 Extended Cab. When I turn the fan/air/heater knob on (on the dash), I instantly hear a noise under the hood. It's not a good noise like a motor running, but more of a whining noise. Any ideas? Don't have much money, am unemployed, so am looking for a way to fix this. Not too mechanically inclined. By the way, no matter if I just have the fan on, the heater or the air, the noise is the same. When I turn the knob off, the noise goes away. Please feel free to contact me directly (mgd@teleport.com). I really appreciate hearing from you. Greg