I used to be scared of the sound of cold water tanks refilling on older-style DHW systems after the hot tap had been run for a lengthy period, especially the high-pitched shushing sound of the water running through the pipes in the entire system during said refill operation.
Ditto for the sound of some toilet cisterns refilling.
Also, open-front type toilet seats used to freak me out when I was little, but luckily I never needed to sit on one to do a number 2.
I'm OK with them now though.