Naturally, if external noise is loud enough to be noticeable, it is distracting and annoying. Sources are anything from ground loops or bad connections to equipment that generates strong EMF like motors and unshielded transformers. Digital equipment sometimes makes a grungy sound.
At times, it becomes difficult to trace these sources and so I normally prefer to get an expert to handle all that. Nothing is as distracting as external noise that cannot be controlled.