I heard the OCZ 700W power supply is faulty which I have so I bought a new Corsair 750W and installed it. When I turned my computer on everything ran fine but there was this really low humming sound that went in cycles, like up and down. So I reinstalled my OCZ and the same sound happened again. At this point I don't really know what's going on unless this sound has always been here and I just never noticed somehow.
Could be a fan that speeds up intermittently. Check the fans on the mobo, the case and the fans of the video card. You can momentarily insert a pencil in a fan to stop it, that would help in localizing the source.