full opp rudder
Prop mod or wavelength is interesting but does not explain, as you say, why it just appeared out of the blue or intermittant.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.
Lots of clues there.
Need to work with what we know, use good radio theory and experience.
When I have had weird radio problems more often than not it is something earthed that should not be, or something that is poorly grounded / bonded/ shielded that should be.