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 is usually about the size of a playing card and often held in place by a plastic rivet.
In some models it is the plastic ceiling.
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.
If you need more specific help, please reply back with your brand and full model number.We're happy to help and we appreciate your thoughtful rating of our answer.
William E. Miller, AS-EET[email protected]