I have had no joy on the Apple Support Forums, so thought I would ask here in case anyone has any ideas.
The header says it all. I have 2 iMac 27 inch machines at different locations. One runs 10.6.8 and the other 10.8.9. At exactly 8.00pm every evening they both let out a reasonably loud "screech". A single noise lasting perhaps 1 second. That is it. They don't exhibit this behaviour at any other time of day and behave perfectly normally. I have run Sonos anti-virus software and no threats discovered. I have not noticed anything odd or different about their behaviour otherwise - i.e. performance is not affected at all. The 10.6.8 is in my office and I am not usually there at 8.00pm. However the other night I was and had just started it up. The only applications running were MS Word and Safari. It too made the identical noise to the other machine.
It doesn't appear to affect anything, but the situation is strange and I have absolutely no idea what might be the cause or what to investigate.
Anyone ever heard of anything like this before and have any observations or suggestions?
Have you tried to set the time manually and see if you can reproduce that "Screeches"? You could also open Activity Monitor and check if a specific process usage increases at 8.00pm… The sound could come from an alert in your calendar or another background application.