I've got a 2001 4.8, i get this noise which is coming from under hood, it's a cross between a hiss and a screech, sometimes it's lasts longer than other times, and it's usually just after accelerating, one thing i've found, is that it doesn't happen when my fan switch on the dash is turned off, i'm suspecting something is about to fail in my AC system, but why only when my fan switch is on, any ideas would help, thanks.

Roland

 

Your belt must be slipping against the a/c compressor pully.

 

yeah sounds like a belt to me

 

the little a/c belt..its tensioner likes to crapcan..especially on dirt-road trucks

 

Thanks for the replys back guys, one thing i'm puzzled with, is that it only does it when my fan switch on the dash is set to one of the speeds, no noise when switch is off, ac belt turns all the time from the motor does it not?

 

sure it does..but its not loaded...just a freewheel

 

Thanks Argon, i've got a belt, guess i'll pick up a tensioner as well and replace them both.

 

Do you have an electric fan?

 

not on a 2001