What causes a noise under the hood when the air conditioning is on in the car?

I'm not a mechanic but these are the culprits that I've experienced: 1. If you turn off the AC and there is no more noise then: a. Your compressor belt may be loose. Check the tension of the belt near the AC compressor. If the belt seems loose a +/- 1/2 inch give then it's too loose. If the Belt is so tight you cant move it an 1/8th inch, too tight.. could damage pulley. b. The Compressor pulley might be bad. Have a mechanic check it. Compressor parts can go bad and they work hard in extreme hot states like Texas. If it's not a belt, I'd pay to have the compressor checked. If you dont then live with the sound and when the pulley dies, deal with it? :)