Thanks
Machaca for the pics.
Location, location ... and an intermodulation product related to the ADF seems now to "have" been the problem.
boguing;
Any chance that the start switch is malfunctioning and staying live?
That could explain when the noise cuts in. The snubbing capacitor becomes ineffective above a certain RPM due to the low impedance of the battery hanging across it. But hang on! If a starter is used, it must be a fairly high impedance "impulse" type, otherwise you could kill the permanent magnet.