has this happened once? or repeatedly?
chances are its the fan bearings - worn out. try and see if you can make it happen again. maybe post a vid to you tube , as we haven't got much to go on...
What was being cooked at the time ? tonight i put steamed veg in sachets in my microwave - it did say on the bag it might make a noise because of the escaping steam... sounded really weird to me but at least i knew what it was, it sounded like a screetchy eeeeeeeeeeeee sort of sound