I have a 2001 4.8 Silverado, i get this screetching noise from under the hood, only when my fan switch on the dash is on, doesn't do it when it's turned off, and it doesn't matter whether the AC is on or not, this noise comes and goes, and happens more often when i'm on the peddle and kicking it down to a lower gear, noise will last up to 4 or 5 seconds, sometimes shorter. I replaced my alternator last year for a 130 amp instead of the original 105 amp, with that, i also replaced the serpentine belt, both were aftermarket, thinking that my noise problem may have been from an aftermarket belt, i replaced it today with a genuine GM belt, problem still there, anyone have any ideas?

 

Not 100% sure, but you might want to check the compressor for the A/C. Just because the A/C isn't on doesn't mean she won't kick on with the blower. (Most of the time it cycles with the defroster). It sounds like the compressor is either slipping on the belt of fighting the belt. Check R-134a pressure and make sure the compressor is cycling on and off, if its on all the time you might be running low pressure. Obviously check to make sure the belt is adjusted tight enough but also check the belt tensioner too. If that is worn out it will allow the belt to slip. (It will bounce around alot if its bad).

 

Take the new belt off and check all of the pulleys and tensioners. Then go throught the paces with trying to narrow it down with what is on and what is off when this occurs. If it is the AC compressor then it will happen when the AC is on and when the AC compressor cycles. The compresoor cylces on and offall the time, and is only on when the AC is on. If it is the blower, then find out when it is making the noise as far as which speed. FYI the blower stayes on all the time even when the switch is in the off position. It is to keep fresh air flowing in the cab. Then make sure that it isn't the cooling fan on the radiator. If it is belt driven then check it when the belt is off, if it is electric you will have to heat up the truck and listen to it when it is running. You can kick it on with the AC, it will come on when the AC is on. Good luck tracing the problem.