It just gets better.
out of sheer desperation we mounted a new antenna on the underside of the fuselage directly underneath the present COM1 antenna. We disconnected the cable running to the antenna on top of the fuselage and connected it to the one underneath the fuselage.
After 2 days the results so far are that the new antenna on the underside does not suffer from the problem (I'm not relaxing yet though)
I can tune both comm units into the same station in front of the aircraft and the one with the antenna on top of the aircraft gets the noise and the one with the antenna underneath the aircraft does not.
This just ruled out the radio or the cable or the antenna as the issue yet again.
Just to clarify a handheld in the cabin does not pick up the noise,neither does the VOR or ADF.
I think we have established that it has something to do with the airframe acting as part of the antenna and some characteristic of the signal recieved from an antenna on the top part of the fuselage combined with some RPM or vibration related interference being generated in the nose of the aircraft.
The new antenna is the same as one that exibits the problem and they are all equally well bonded, it is just the physical location that has changed.