cold start up high pitch squealing noise from engine

Check the fan clutch. That's what it sounds like to me. It may need to be replaced. They cost $50-60. I've never tried to take one apart or fix one, I just replace it.

Belt squeal because it's slipping would be my guess. The belt tensioner is probably wearing out and not putting the tension like it used to and the moisture in the air when it's cold is making it slip

I know this is an old post, but thought I would add anyway. My 2007 2500 started doing the same thing last year. Especially on cold starts. Very soon it started having shifting issues. Took it to mechanic and found out the torque converter was going bad. $850 to replace, but that was the problem. Mechanic knew it as soon as he heard it

My 2003 chevy silverado 2500 does it only at start now faintly, but nonetheless still embarrassing. When it first started it did a few times while running in park, then I stated it the next day and it would not stop then 2 days later only when I started it for a few seconds, but noticed a faint nonstop ticking. Had drive train belt and ac belt replaced for $200 yesterday and started it today and same sound ‍♂️. Runs fine shifts fine no other noises but tjay squelch at start up for a few seconds.

I have a 2006 Chevy Silverado 2500HD 4x4 6.0L and was squealing when started and did get louder when stepping on the gas. changed belts and upper tensioner pulley and it still made the noise. removed serpentine belt and A/C belt and started truck and noise went away. changed A/C tensioner pulley and put all the belts back on and noise is GONE!!