I had the exact same problem with my 450 SP, and I replaced it. It was at 12.4V, not great, but not critical yet. It also made the squealing noise. I replaced it with a 750W unit. Did you overload it in the past?
CPU: Intel C2Q Q6600 @ 3000 MHz
RAM: 2GB DDR2 667MHz
Motherboard: Asus P5K Deluxe
Video card: BFG 8800 GTS OC