I have a 2007? 27" Imac (all one piece, flat base). Am not that computer-literate; so any info will be helpful! Have had fan noises before, some clickings on and off; all resolved. But this is a weird sound-like a fly caught inside the machine, buzzing every 15-20 sec., lasting 3 secs, but all the "hiss/buzzes" are not the same in tonality or length.
. Sounds like it is coming from lower left side, in the back. Loud enough to hear across the room. It just started last night. REALLY unusual-it does it whether the computer is on or off! Any suggestions? We are due for "cleaning, de-everything, and updating"-done by a "Mac Pro". Thanks! 