Its probably coil whine, the Seasonic X series have a problem with that, as you can see on reviews at Newegg. Call their office and have it RMA'ed.
I think I'll call them...too bad they don't post their hours. I sent three emails. Two Wednesday (different accounts as I heard they block certain domains), and one today. Yes with regards to the newegg reviews, although mostly on the 750, but I'm fairly certain these two PSU's are virtually identical. It's just disappointing when you spend $135 on a PSU and this happens. Should have gone with the XFX 750...although I think it too is made by Seasonic.