I'm wondering if anyone has brought this up yet, but this sounds similar to an issue I've experienced with PC's -
COIL WHINE
Basically eminating either from the caps/coils in the PSU, or on the motherboard.
My last PC, before I switched, would make beeping/chirping sounds dependent on CPU load, and particularly dependent on graphics load. ie. it would be silent if the screen was showing just the desktop (with zero motion) and would chirp and whine if you were to drag a window across the screen.
Occasional topic of conversation over at
www.silentpcreview.com I know they've chatted a lot about the mini (quiet) and the PowerMacs (loud wind tunnels) but I haven't seen anything on the new iMacs.
Need to start looking I guess, as I just ordered a 20" last night.
edit - guess I should have added - while it may be noisy, it doesn't mean the component is defective. SPCR maintains a list of noisy mobos, and we tend to stay away from them. There are some messy mods/fixes that usually include coating the offending coil in melted wax, or hot glue, to reduce/eliminate the resonance that leads to the beeping/chirping