..and now after 3 or 4 wasted hours I've solved the problem.

The studio quality headphones connect to the machines
via a stereo full-size jackplug to mini jack converter.

There must have been a slight misallignment that was cancelling out
the audio from the discs that weren't working,
at the same time creating that weird 'rodent like' signal distortion.

But I was still perplexed why some recordings were still playing correctly..
Until I realised that the 'faulty' discs were all mono recordings,
while the discs that all played correctly were stereo.

Somehow, I guess the mono signal must have been affected by some kind of phase cancellation maybe ???

A small accedental tweak to the jack connecters while swapping from one machine
to the other obviously restored a good true audio path.

oh well, live and learn !