I'll add an update and answer some more questions.
Firstly
FullOppositeRudder has correctly identified the aircraft, well done! That photo is a few months old and she is now sporting a new front antenna which is one of the large angled fibreglass units, mounted with 4 bolts, and perfectly bonded.
As far as I know the aircraft was never exposed to any lightening. This problem started for no apparent reason on an ordinary day and due to it's intermittent nature is extremely hard to correlate with anything. We have investigated the possibility of a magnetised crankshaft or something else, but everybody seems to agree that this would always have the same (or similar) effect. It could not start and stop.
During the last few days the problem has been present for about 50% of the flights. On one day it was there from cold first thing in the morning and then mid afternoon it stopped during the peak heat of the day (37'C)
On another day it was not there in the morning and started in the afternoon. The following day it was not there all day. It absolutely does not coincide with variations in temperature or humidity, it seems completely random.
Each time I have had time to troubleshoot the issue it seems to stop and so I have not been able to disconnect the coax to the main antenna to see what happens.
When I listen to the audio on the VOR when I am having the issue it seems to be clean, however the VOR antenna is mounted high on the tail and as I am unsure as to the directionality of the VOR antenna or the source of the transmitting station I cannot be sure it is not affected.
The noise is the same whether listening through any headset point and when bypassing the intercom, there is no speaker fitted.
When the problem is there breaking the squelch makes no difference. The noise is there on all VHF channels when receiving a transmission, if pointed straight at the transmitting station.. I have not tried tuning to an unused frequency and breaking the squelch to see what happens, I will try that next time I fly.
I'm leaning towards the dodgy diode/capacitor/connection somewhere in combination with vibration being the cause, but I am running out of places to look!