My iMac recently developed a slight annoyance. It's making this....chugging/tapping noise. Almost as if you were to tap your fingers on the desk. It's very faint, but still very noticable when the volume is down. It started just recently. I shut off the system and turned it back on, and it was gone for about a day, and now, it's back. I can't tell where the noise is coming from. I don't think it's the fan, but I don't know what else it could be? Anyone else experiencing this? It's driving me crazy--kind of like the chinese water torture.