Dont know if this is valid for nforce3\4 or\and 64 bits systems
On my 8RDA6+ this noise is caused by the stop grant disconnect feature (on by default, and not accessible yet through non modded bios)
I use speedfan(advanced chip config) to turn it off\on.
Have experienced similar noise on boards from asus, asrock and kt133a boards from abit.
Sometimes the noise would only be generated when pci cards were active fex modems or sound cards.
Seem to recall the author of vcool explaining its some sort of mechanical noise generated by the cpu when disconnected\connected to the bus. Dont know if this valid any more though.