Quite often this is due to grease or food particles stuck to the inside of the oven or stuck on or behind the waveguide cover.
The
waveguide cover (also called a spatter shield) is usually about the
size of a playing card and often held in place by a plastic rivet.
In some models it is a larger stirrer cover type made of thicker plastic in the ceiling of the oven.
It can be fragile, and the metal edges of the oven behind it can be sharp, so be careful!
The cover and the metal behind it should be cleaned with a mild, non-ammonia cleaner.
If the cavity metal of the oven is chipped or charred, it will have to be sanded smooth and clean and dried thoroughly.
Then you can apply some high-temperature appliance paint, available from places like Wal-Mart, Lowe's or Builder's Square.
If your waveguide cover is burned, it will need to be replaced. It provides important protection for the magnetron.
The best place to order Sharp parts is
Tritronics.
We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.