2000 K3500 with the 7.4L vortec engine. Recently when starting the truck I would get a very high pitch whistle noise off and on only for a few seconds when cold out. I've noticed this noise only comes when it is cold outside, less than 60 degrees. Above that temperature it doesn't make any whistling noise. When it does make the noise it is so loud it is hard to pinpoint where it is coming from, and after about 3 minutes it will go away and wont come back until the engine is fully cold and started back up. Anyone experience this?

 

Does the pitch change when you rev the motor? Belt squeal??

 

It does not. I am thinking it is an Idler pulley and will change that out today. Hoping its not alternator or AC compressor bearings. Will put in a new serpentine belt as well.

 

You can use a piece of heater hose as a stethoscope & carefully place it on each component (pulley, a/c, alternator) to find where the sound is loudest. CAREFUL with hose around moving belts & pulleys!!

 

Confirmed Idler pulley bearing was bad. $25 later and a new serpentine belt the old 7.4L purrs like a kitten with no noises.