Just yesterday, my friend's '08 Silverado (LY2 4.8) started making a squeaking noise that sounded like a belt or pulley. Not too loud, but you can hear it from over 20 feet away.

He removed both belts, and the noise was still there. Removed oil cap, and the noise was still there. If you put your ear next to the oil filler with the cap off, you can hear the squeaking coming from inside the engine.

The noise speeds up with RPM, but disappears at about 1,500 RPM.
Oil pressure is good (30+ psi) according to the factory gauge, but I can check it with a mechanical gauge if necessary.

We decided to cut the oil filter open, and the filtration medium was CLEAN.

Any recommendations on where to start looking?

 

Cam retainer bolts hitting sprocket?

 

Bearings

 

A squeal or squeak is “usually” a lifter turned on a cam lobe, or a lifter wheel crapped the bed. But you would see something in the oil filter…

 

Well, it had just started when he contacted me. Maybe it hadn't had enough time to fill the filter with shavings.
I installed a magnetic drain plug the same day, and we're going to check it Saturday (when He's back in town). I'll see what kind of debris (if any) is on there.

I forgot to mention, this truck has less than 80K miles on it.

 

Okay, we checked the plug today, and it had some small, but lightly gritty black debris on the magnet.