This is gonna sound odd but one of my biggest fears are... extractor fans. The kind you find in toilets?
I don't even know what set it off in the first place but since I was 3 I've always hated the way they look and the way they sound. Every time I go into a toilet I've never been in before, I'll always stop and look for a fan and if I see one I either freeze or run out of it. I gotta go into that same bathroom like 5 times for me to be comfortable.
Another fear is toilet related and it contributes to the whole fan thing. I am terrified that I'll be in a bathroom which has a fan in it and someone outside will turn the light off, I dunno why but I find the thought terrifying.