P&D replacement to the rescue. In the box are some spare axle/gearboxes. Simple process to pop the old one out and pop a new one in.
It's reduced the rattling significantly. Now if I run the sound decoder, I can't hear it.
The new motor Pat supplied has improved the decoder performance, but I'm going to have to upgrade to the Loksound L version to get rid of the groaning sound completely.
I might set Pat's chain drive up with an idler to replace the old one to see if I can eliminate the rattling completely.
Sheesh! This O gauge lark is a challenge!
Cheers