MM43 wondrs:
Going back to very early on in the thread, I did mention that a zener diode could, if reverse biased, generate the "white" noise. It seems that condition could exist in the alternator regulator when current to the base of the pass transistor is cut off. A capacitor associated with the zener will need to be open circuit for this to happen, certainly a possibility, and something that a standard regulator test would not identify.
Zener or other electical white noise source fails to explain why the interfenrence when pressent is at the same relative level over a 40Db or more Rx signal level range.
(HF did stat 10+ miles range so my 200M to 20Km range is valid.)
From HF original post, a couple more reasons why it is very unlikely to be an external (electically generated) RFI problem:
Idea 4, the alternator. Engine run with alternator field disconnected (so not energised) and master bus disconnected. No Change.
Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change
When looking for caues of a bizzare issue such as this it is important to keep all the clues in mind, even then it can get pretty confusing.
MikeW: Yes I knew that the pistons go left right just let fingers get ahead of brain.
Even was part owner a VW bus once until one of its pistons ended up goind in/out and then allover the place...