I've just had to restart my iMac and it powered down OK but then just sat with a black screen for about a minute then all the fans seemed to come on at once at full tilt. I let it run like this for about 2min. before using the powere switch to shutdown waited a min. and started afresh with no problem. This is the 2nd time I've restarted the PC and both times this has happened, is it normal should I leave it longer with the fans in 'launch mode' to let it restart itself?