sharing same engine and likely shared same squeals.
here's what I had done:
1. idler pulley
2. tensioner pulley
In that particular order.
Pop hood open and start the engine.
Cut a suitable length of garden hose and use it as stethoscope. Just be reasonably common sense cautious around moving parts.
#1 on the list is single bolt mount. Very easy to replace. #2 is same. #3 is about an hr labor DIY and $100 or so.
You have serpentine replaced, right? It's due at 100 000 miles. Tensioner has markings on it showing if it's stretched or not.
2000 SILVERADO 1500 CENTURION EDITION, 5.3 VORTEC
DON'T FIX IT IF IT AIN'T BROK;
IF IT'S BROK, FIX IT RIGHT AWAY.