I once left a stove burner on in the evening,
nothing on the burner.
I woke during the night and thought I heard the exhaust fan of the mw
going on and off. I
decided there was a burglar in the house trying to terrorize me and
finally marched out to
confront the burglar ) Dumb! No burglar. Apparently the heat caused
the fan to cycle
on and off ..... I have no idea whether it was designed to do that or not?

That's normal. There is a hood thermostat that detects the heat to
protect your microwave from overheating or steam contamination from
the stove.

William E. Miller, AS-EET

http://www.microwavedisplay.com