High pitched sound on overclock - problem?

A few months back I built a computer with the following spec:

Q6600
P5K-E
Antec P150 case (with 430w PSU)
EN8600GT Silent (passive cooling)
Transcend DRR2 800 RAM
Samsung HD501LJ Hard drive

The computer has been working fine. I had it overclocked to 3GHz until recently when I overclocked it further to 3.2GHz. (1.4v)

The memory is rated for 5-5-5-15 with 1.8v but I am currently running it at 4-4-4-12 with 1.95v. It has been running at this latency at both 3Ghz and 3.2GHz.

Since the overclock to 3.2Ghz the computer has begun to emit a low high pitched sound usually under load, but also sometimes on the movement of the mouse while highlighting something. The sound is low but high pitched, tinny and electronic - almost like the noise the sometimes comes out of monitors.

I've listened inside the case to try and locate the source of the noise but its pretty difficult to pinpoint. it seems to be either the video card or the MoBo.

Temps on the computer are pretty low now summer has gone. And the computer seems to be running ok (except for one issue, where Spy sweeper is suddenly taking 3.5 hours to run a scan that it used to do in 15mins - but not sure if this is related).

At 3Ghz the sound almost disappears but i have began to hear it a little at this speed too, although only slightly.

Anyone any ideas what the problem could be and if it is serious?