Key clues to my mind:
- Non-airframe grounded receiver does NOT 'hear' noise => Noise is not an RF transmission
- Opening squelch on the a/c radios on an open frequency does not 'hear' noise => Infers something is happening to the transmitted signal BEFORE reception by the a/c radios (and reinforces this is not an additional RF signal)
- Only occurs to stations when within +/-20 degrees of being on-the-nose => Something in the front of the a/c is responsible! (Thinks big whirling thing out front...) What angle does the prop diameter subtend to the tower station at 200m. Does the angle of interference appear to change?
- Noise is not always present => See post above by MurphyWasRight! I'm with you on that idea.
Back to thinking some more
- GY