Normally I would say that it is impossible for a vga to kill the psu but in your case I'm not sure anymore...
This sound normally appears in the field of high power consumption.
Could it be that it was there with the 4870 X2, too but you just couldn't hear it because of the noisy fan?
It could happen that the windings in a coil (normally fixed together) are free to move a little bit.
So if the 4870 X2 has damaged the psu by demanding too much power this could have happenen.
I am not sure about your problem at all - sorry ...
Xien16