Update:
Today we put two bonding straps between the airframe and the elevator, and a test flight shows
the interference has totally vanished.
The interesting thing is why the interference did not occur on the flights done since the Annual in January, and those done more recently when it was noticed.
For example I did the JAA IRT during that time, and definitely used both radios on that. I also did some airway flights, and likewise.
The answer may be that the conductivity of the two elevator bearings was intermittent, but it could also have been in the amount of water droplets shed by the aircraft. On the day of the
IRT the temp/DP was c. 08/02. On the
day I really noticed it it was c. 07/07 and thus a lot more water droplets were about.