Yes, I've been wrestling with all the possibilities for this one also in my spare moments. It continues to elude a simple 'can't miss' diagnosis, and is made all the more frustrating because of its intermittent nature. The points raised in previous posts are worth checking.
I find it hard to latch onto the fact that it occurs only in a narrow RPM range, and that as 'improvements' were made to the ignition systems, the RPM trigger point came up 300 rpm from the previous one. Can this be linked only to vibration, or is there some other factor which causes the noise to appear at that specific figure? Are there other instruments or monitor systems which come into play at that number?
Do the antennas have some sort of static grounding system at the feed point which has reduced the overall RF sensitivity so seriously as to make the direction of the signal source so very narrow and specific? An SWR check would normally betray this, but lets not forget that a 50 ohm resistor at the other end of the coax will quite probably return an excellent VSWR measurement. Is the noise equivalent to an open squelch level and character or is it 'louder' than that?
There's one other factor which the rest of us need to keep in mind. While it hasn't been spelled as such, it seems likely that HF is working at this by himself as both fix-it man and pilot. If that's the case, it adds additional complications into the whole process because flying the aircraft is always the absolute priority, and trying our suggestions will - of necessity - come much further down the list in any given exercise. We all need to be patient as I'm sure he is, but we all share in the frustration which this problem is causing, There has to be a fix somewhere!
FOR