Ive gone through a rash of late 2009 iMacs and three of them including my current 27 iMac i5 make a ticking noise. Its a quiet noise, the interval is about 25 seconds, and its hard to pinpoint where exactly its coming from. I dont think its a mechanical component since it keeps making the noise after the machine is off for a minute or two. My only thought is that its related to something expanding in the system as it warms up and cools down. I have a Playstation 3 that made a similar noise for about the first six months I owned it. Anyone here having a similar experience?