Do it show up when gaming? Load up a game and then put your ear to the psu vent. If it is from the psu you would only hear the coil squeal very clearly from the power supply. If not then it will almost certainly be the card especially when it changes tones depending on changing fps.
So what model psu are you using and what brand as well model card are you using? Certain cards will do this right out of the box due to the inductors used. Ferrite shielded inductors have the buzz issue more than any other coil type and they have much higher failure rate.