interesting notes. yeah it happens everyday in the evenings. a electrical buzzing voltage sound that is loud (i can hear it about 5 feet away) i did purchase voltage regulator. this one to be exact:
http://www.amazon.com/gp/product/B00009RA60/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER
And still the buzzing occured.
Also i recorded another sample of the buzzing noise here:
https://soundcloud.com/pavl-1/imac-noise3
I also had a similar issue a while ago - testing the open iMac in front of me, the technician showed me the PSU making that "frying coil sound", especially when under heavy load. I changed it for 200 bucks and all is fine now.