Well no solution found today.

I am pretty certain it isn't a mechanical issue. I did discover I had a faulty limit switch on the X axis home and thus replaced it (in configuration I saw the switch flashing on and off when the machine was well away from home).

The workshop where the machine is located has its own electricity supply so I think outside interference can be excluded?

The parallel port I am using is a StarTech 2S1P PCI Serial Parallel Combo Card and this was replaced with a new one to no avail.

One thing I noticed was after changing the parallel port card and powering the PC and router bakck on I jogged the router in X,Y and Z almost full travel several times without the problem showing itself, then I sent the machine home after which the problem noise was back again. Maybe coincidencebut it makes me wonder if something is on it's way out and as it gets hotter it starts failing?

I am now putting an image of the working PC that was taken a few months ago onto a spare PC to test on Thursday - at least now I have a parallel port card to use in it.