For Halloween I want to haunt my toilets and have them make the noise heard here: https://www.youtube.com/watch?v=6S0zewtkqso
How can I do that?
I think I would just need to cut a slit in a rubber part of the fill valve.
Sounds like a pissed-off fill valve, acting like a reed. You'd have to get your hands on a crappy toilet, swap out the fill valve, and hope it has a similar failure; not recommended. (all puns intended)