Sometimes, though not constantly, my Imac will sound a loud bell. I have wireless keyboard and mouse. There is no way I can figure out the cause. It comes and then might skip a couple of days. Could the mouse be the cause? Incidentally, I did call Apple and all they could suggest was to keep an anecdotal log for future reference. It is a most disturbing sound and appears to be some sort of warning. Help would be most appreciated.