My toilet is whistling. It does not whistle during a flush, or while filling it up.
It does whistle when I shower, when the washing machine is used, when the neighbor downstairs uses water, and randomly at night. I changed the flapper but it still whistles.
Whistling began after city came and changed the main water meter (I live in an apartment).
Building is 50+ yrs old. Toilet was likely replaced in 90s.
Any ideas? Thank you...

Im guessing that when the water meter was changed, some debris go into the toilet fill valve(very common, no one to blame, it just happens sometimes). I would start by cleaning the insides of fill valve, or simply replacing the complete fill valve(usually a cheap and easy fix). This should solve your problem.

One thought, just to make double sure. Close the shut off valve under the toilet, and then run the other fixuters and see if noise is gone. If it is, its definitely the fill valve in the toilet that's causing your problem. Good luck and please let us know how you make out. Lee.

Mygirlsdad it nailed down, I will also add the image below if you tell us which fill valve looks like yours, Thanks.

John