I would replace it.

Is this a new MW (still under warranty) or one that is several years old? IIRC, it was two or three years ago that GE and Whirlpool each had a spate of MWs randomly turning themselves on. There was another spate of that about a decade or so ago, as well. Try googling "microwaves self starting."

Could be a bad diode or a bad capacitor. It should be checked out right away or if it is a countertop M/W more than 6 years old replace it.

Thank you, jwvideo and Jakvis. I appreciate someone knowing about these things. It is a GE, and since it's more than 6 yr. old, I'm going microwave shopping today. I have not used it since the noise, and have found that I really only use it for convenience, not because I actually need it, however, I do miss the convenience.

The Bosch website says this type of noise is normal from microwave ovens: https://www.bosch-home.com/ne/service/get-support/microwave-making-noise

Thank you, J Smith. It has since been replaced.