Took one look at your sig and can tell you it's the power supply. 500W and your running 3 HDDS, the GTX 260 and a decent cpu. I run dual GTX 260 Superclocked cards and if I remember they recommend 600W for one, of course it can be lower with a decent PSU and IMO the Antec TP 500 is already stressed before it gets close to 500W; I have one running on my media PC right now; and that is most likely the cause of the whining you hear too.
I run a Corsair HX 750W in my gaming rig, not cheap but proven stable at 850W in many reviews.
Hope you get it all sorted.