1999 Pontiac Grand Prix alternator

A bearing is starting to fail. Eventually the generator will lock up and stall the engine. There are repair kits for these that include new bearings, but getting this unit apart is extremely difficult.

This is a very poor design that develops a lot of voltage spikes. Those can damage the internal diodes, voltage regulator, and interfere with computer sensor signals. The battery is the key component in damping and absorbing those spikes but they lose their ability to do that as they age. It is common to go through four to six replacement generators in the life of the car. To reduce the number of repeat failures, replace the battery at the same time you replace the generator, unless it is less than about two years old.

Dandy. Put a new battery in there so your replacement generator doesn't fail. GM had probably the world's second best generator design between the early '70s and 1986. Their voltage regulators work the same way as yours, but they don't have the same problems. Your old battery will work fine in any of those cars until it dies a normal death at about five years of age.