I figured out that metal lens adapter rings cause the noise.
It's not the fan, but the noise comes out of the fan vents.
It sounds like some kind of electronic short or alarm.
I was using M42 to EF and Olympus OM to EF lens adapters.
When I took them off, and turned the camera on, there is no noise.
When I attach a native lens, there is no noise.
When I put a dumb metal lens adapter on, there is noise.
I like to work with vintage lenses, manual primes. They require adapters.
I did find one M42 to EF adapter that doesn't trigger noise.
Several etailers sell it on the AliExpress marketplace:
https://www.aliexpress.com/item/1005001690804307.html?spm=a2g0s.9042311.0.0.406f4c4d2qEiRU