I had the same thing happen and thought I was going crazy... In my circumstances, my sims were in the big Medina Studios apartment with Salim Benali as a neighbor. I installed 2 of the expensive talking toilets and played the game as usual.
After a pretty long time of playing the game, I started hearing a faint humming sound that drove me nuts and I isolated it to one region of their apartment (kind of on the border between my sim's apartment and Salim's). I exited the game and came back in, the noise was still there. I went into Salim's apartment to see if it was something he was doing but it disappeared. Sent my sim back home and it was still happening so I assumed it was the sprinkler system I installed in their home. Deleted it, noise was still there. Bulldozed the entire apartment, humming noise was still there. The only thing that seemed to fix it was deleting cache files and repairing the game with Origin.
I didn't know it was related to the toilets at all but it makes sense. This happened at a time when mods were disabled in my game since it had just been patched for Seasons. Idk if it helps but the noise got gradually louder and louder to the point where I started to hear it more than Sims talking to each other or the radio. It was also very isolated so if I moved to another side of the apartment it got quieter.
