I've noticed on a few threads that a recurring criticism of geared motors is the noise generated by the gear train. I can understand this in the case of an electrically driven replica of a steam prototype.
However, it's been my experience that the overriding sound of most diesel tugs - smaller ones in particular - is the whine generated by the transmission (along with similar noises from turbochargers/hydraulic pumps etc). Usually this is far more noticable than the diesel throb. Maybe we should consider gearbox noise as a free sound effect. Certainly more realistic than some diesel sound generators I've heard!