This has happened twice in my game. For me, it's only when a child chooses the "use and massage". I've been able to stop the sound by shutting down the game and then restarting. Maybe the "massage" option should be removed for children, I don't always catch them before they auto select it.
Confirmed: Tested it on the Brindelton Bay 50x50 lot on 5-27-19. It is isolated to the (use & massage) option for "Grade School" aged characters (needed 2 tries of use before it happened). The hum stayed even after deleting the toilet. It did not happen using a high school aged character or an adult. There were no upgrades applied to the toilet during this test. Quitting and relaunching the game got rid of the hum. This was tested without any mods/scripts loaded. bb.moveobjects was used to insert the toilet in that cubby (see screenshots).
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