High pitched noise from Shower
My tub spout would make an awful squeal when the shower stopper was pulled up (air in the line trying to escape the poorly seating valve). It would change pitch and stop sometimes if I jiggled the lever right. Find the offending unit and ask to replace their tub spout. Tell them why and they will almost certainly agree to it. A change in pressure may get it to stop momentarily. Take a shot in the dark and cycle your faucet valves on and off to attempt re-seating the seal of your neighbors' tub spout stopper valve, when you hear it howling. Or be proactive, finding out whose it is and whose job it is to fix it.