The culprit

 

I hear this sound all day behind my bathroom wall.  Like ball bearings churning. Chuga chuga chuga. I call my super and ask him to check it out and I email Lexi above me to ask if there’s anything weird going on in their place.  She replies there’s nothing unusual.  I text super David and tell him to let himself in over the weekend if he wants - I’m going out to the beach.  A short time later he shows up and looks perplexed and he’s never perplexed.  He can diagnosis and fix anything.  Finally he grabs my battery operated toothbrush thats on my shower thing holder.  It’s running.  He turns it off.  The mysterious sound stops.  I am embarrassed and he leaves saying - “at least we didn’t have to bulldoze the wall.”