I agree that the "grunting type sound" is more annoying than affecting the actual running of the locomotive - it is however most aggravating when the rest of your locomotives run smoothly and silent.
To check, the offending 614 DMU was placed up side down, and using two electrical wires from the 12v dc variable controller, a wire was placed on each of the opposite powered bogy wheels - the DMU ran in complete silence. However it was noticed that a blue coloured line of sparks were seen - this fault is usually related and caused by the motor brushes requiring to be renewed.
The 614 DMU was again placed on the track and 614 DMU set in motion - "grunting type sound" was again clearly heard.
If anyone has heard this or similar sound from their locomotives, it would be most appreciated, to hear how this fault was fixed.