My 2002 GMC Sierra 1500 whistles since having a new alternator and fan belt installed?

Had the work done yesterday and from the first time i started it up I noticed a whistling type sound that I could not pinpoint. The sound is not loud but clearly audible and was not present before the alternator/belt were changed. It:



-is audible from outside the truck

-is audible from inside the cab with the heat and radio off

-increases in pitch and fades away when accelerating, and is no longer audible after about 20 MPH

-If I push the windw buttons up (with the windows already up) the dashpannel and headlights dim, and the voltmeter drops from 14 to 12 then slowly goes back to 14 after a few seconds. This seems new to me but I am not sure that this never happened before.

-Also when i push the window buttons up the whistling noise changes pitch when the buttons are pressed and when released.

-The oil pressure also increases when accelerating. I also dont know if this is normal or not but it is a new observation i have mad since paying closer attention to that side for the instrument panel.



Is it common for this to happen for a few days after replacing the alternator or belt?



I have drove about 120 miles and the sound is still present.



Any help would be greatly appreciated.
My 2002 GMC Sierra 1500 whistles since having a new alternator and fan belt installed?
sounds like the alt is bad unplug it and restart is the sound gone

possible wrong cooling fan on alt installed
My 2002 GMC Sierra 1500 whistles since having a new alternator and fan belt installed?
GMs always do this...their trucks arent meant to be work trucks...more like car trucks....
Spray WD40 on the fan belt.
its a gm thats what they are good at doing breaking