Crack sound followed by repeating three beeps.

Member of the family using our iMac bought in September 2010. Computer on for around 10 minutes when she heard a 'crack' like a stone hitting a window. The iMac then startted bleeping loudly - 3 beeps, pause, 3 beeps and so on. She switched it off at the mains. Switched on around an hour later and seems fine. Wondering what the problem might have been and how to avoid in future.