Another idea - Is it possible that the chimes don't ring until the visitor releases the button? Something happens when the button is pushed down, such as the talkback system turns on, or the relay clicks, or some part of the system engages which emits an ultrasonic whine, which continues while the button is being held down. Then, only after the button is released, the chimes ring. And maybe sometimes the visitors are just being enthusiastic about holding down the button for a long time? This is something you could test easily.