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The GM V-6 and V-8 engines have tensioner pulleys (to maintain constant, consistent pressure on the serpentine belt) and also idler pulleys (to provide a glide path).
It is very common for the idler pulleys to fail. There is a bearing race seal on back side which fails, and the grease slowly migrates out. It is not repairable, and must be replaced.
Remove the serpentine belt, then remove the idler pulley (most have a 15 mm nut). I have replaced many on GM cars - do not “cheap out” and buy a China-made, aftermarket part. The ACDelco parts are very reasonably priced, are high quality, and last a long time.
When you remove the idler pulley, look at the back side. You may see streaks of dry, black grease - which indicate a failed seal.
If the tensioner or idler pulley gets real bad, they can cause the pulley to seize, locking up the serpentine belt and ruining it (and stranding you).
The tensioner pulleys are replaced in the same way, but usually have several bolts holding them. If you watch them while the engine is idling, you will see them shake and vibrate slightly. That wears out the bearing. If you remove the belt, you can use your hand to push on the tensioner, to hear if it is squeaking. This is not a fail-safe method to check it. The tensioner pulley can be bad, or the spring-loaded tensioner (or both).