Typically, if you have a squeak from the engine on the C5, it’s the harmonic balancer. When the car is idling, see if it’s wobbling at all. If it’s wobbling, the balancer is separating and the belt is squeaking from the wobble.
Mine had a squeak, I used lots of different dressings to stop it temporarily until I was going to replace the balancer. Still haven’t replaces the balancer, but installed a GoodYear Gatorback belt, and no more squeaking!
So, long story short, change to the gatorback belt and see if this fixes it.