I just moved into an older rental home and during the middle of the night, a high-pitched noise started and increasingly became louder. It was not constant, but would stop for a few seconds before starting up again. This morning, I tried to find the source of the sound and it was loudest in the wall where the shower head is mounted, which is also the wall behind the toilet, and I did not thoroughly check to see if it was coming from near the toilet instead. After I flushed the toilet, the noise stopped completely – at least for the rest of the morning.
Suggestions on things I should check for would be appreciated!