PSU Beeping/humming

So my power supply, a Rosewill Green series 630w, has a very faint beeping or humming to it. I have had my computer built for about 2 weeks now and i am only just starting to notice it because i turned the fans down in my system. It only beeps when i am loaded into windows 7 and only when i have little to no stress on the computer. (idle/minor web surfing) It goes away whenever i put load on the system such as when i play a game or start up prime95. Not sure why the sound is there...