It took DS5 a very long time to get over the fear of accidentally switching on the bathroom fan. Even now he has to turn the hall light on so he can see the switches. And if DH leaves the fan on in the bathroom, DS5 won't go near it.
Is there another light nearby that he can turn on to illuminate the switch in the bathroom? Then maybe you can clearly mark the correct switch and he can practice until he feels comfortable.

Or ... tape down the fan switch so that if someone wants to flip it, they'll have to work at it? It would still take time, but maybe he'd be more confident in himself.
Bless his little heart on worrying over the ding from the microwave.
Ahh, bless his heart. Thanks for the idea about the hall light switch. It's right outside the bathroom!! That's the next habit we will have to work on!
When he was going to a psychologist, she did more harm than good. She gave him homework of trying to desensitize him by having him flush the toilet multiple times a day. All it did was stress him out more and he began to have accidents, which he hadn't done since potty training days.
But it was during that time that we discovered the light switches were just as much a problem as the toilet flushing. I drew him a diagram of the lights and fan so he'd know which light to turn on. Unfortunately, from bathroom to bathroom (we have 3) it's not the same. It was too hard to remember, and he was too anxious to think clearly at the switch when we left the diagram there.
What he is doing now, is coming to get us when he has to go to the bathroom. He will walk behind us, until WE turn on the light. Then once the light is on, he wants his privacy! It's reduced his stress and there are no accidents.
Toilet flushing is another biggie. He will not flush the toilet. He is too scared. Even if I am in the bathroom with him and I flush the toilet he will run. He has hit his head twice on the door knob because of fleeing and not being aware of his surroundings. There is a rug at the bath tub. If he stand on that with bare feet, he can handle the toilet flush. But not standing on the ceramic tile. I don't understand that. But if I am going to flush and he is still in the bathroom, I tell him to stand on the rug because I'm going to flush. I guess the feel of the rug on his bare feet distracts him enough to handle the flush?
Bathrooms at Disney can be a disaster. We carry post-it notes to keep the toilet from flushing on him. Companion bathrooms work much better since there are no neighboring toilets flushing.
For the vaccuum cleaner and the garbage disposal, we have to tell him when we are turning it on. He will go sit on the couch (his refuge) until we stop using it. Again, maybe the feel of the couch fabric on his skin distracts him?
Airplanes and helicopters send him under the kitchen table.
I'm open to any advice that will help him to tolerate these noises better! So keep it coming.
Lurkers, you too. If something has worked for you or your child, please let me know.