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The part about the noise being low at sea level and very high at altitude
makes me think it is related to the ignition system. As you climb the
thinner air provides less of a dielectric in the distributor portion of the
magneto. This increases the ionization of the air inside causing more EMI a
la Marconi. I suspect that your P lead is improperly grounded. P lead wire
should be shielded MIL-C-27500 (I used 18 gage for durability, ACS PN
11-14418) with the shield connected to the magneto case and the magneto
switch ONLY. If you are using shielded wire that is grounded only at the
source then the P lead will radiate LOTS of EMI.

I would:
1) Verify P lead wiring
2) Try another radio (radio may be shifting frequency with altitude)
3) Land at high altitude airport and turn everything off except radio and
Tx. Noise?

My .02

Regards
Brent Regan

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