After spending a few hours checking wiring, earthing, screening, and disconnecting other equipment, I came to the conclusion it was the V16 itself.
As I removed it I heard something rattling inside, opened it up and found an Al electrolytic cap (C34) had broken loose.
Its tabs had fractured at the 90 degree bend, almost certainlydue to vibration and probably not helped by the V16 being mounted inverted so any bulky components are hanging by their solder tabs.
And probably also saved by the inverted mount so the loose cap metal can didn't short out other components.
Replaced the V16, this time putting dabs of sealant on each heavy component/pcb interface to help prevent movement under vibration.
Time will tell how effective this is- better would be conformal coating over the finished assembly.
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