Power Supply of my home PC: It has noisy for two days; now it's not

Something funny with the Power Supply of my home PC. It was noisy for the last 2 two days with some clinking sound kept coming from the fan part of the power supply, as if some metalic thing got stuck and clash with the fan all the time. I was very close from going to computer store preparing to shell out HK$400 to buy a new 450W power supply in order to avoid the annoying sound. But the morning I woke up and found that the noise was gone. It was reasonable quite as usual. Now I am like confused, wondering what was going on in the last two days.