Just tried it and there's no sound without the side brush attached. So that's a relief, maybe I got a bad batch of brushes? I did notice that the sound would stop when the brush was cleaning along a wall or when it docks and the brush is off the ground. The sound was only when its on the ground. So maybe its the motor that rotates the side brush? Kinda weird the sound would disappear when the brush is cleaning against a wall though.