Hello , i have the very same PSU and have the same problem , it's the fan bearing making the noise , either RMA it , or replace the fan which will void the warranty , in my case RMAing it would cost me the same as buying a new PSU so i just bear with it , it doesn't really bother me because i'm mostly using my headphones .
Many peoples with good rigs from what i see have series Corsair RM. They is bad really?
Its overrated overhypend and overpriced you can get much better for the same money or less, only reason everyone buys it is because they dont know any better.
Its not worth buying unless you can get it dirt cheap.
You dont need 750 watts unless you are going to have 2 video cards a 450 to a 550 watts is more then enough as long as you dont have more then one video card.