[chrisktx]

Just wish I had more time...anyway...

I have learned that squelch (by default) is always on, when you pull the squelch button, it actually turns it off. Except on 121.5 on Narco... I know, this discovery sounds freshman... but least I know the squelch is doing its job...its the 'raw' radio signals coming in

I've also believe that the avionics master in this particular plane was wired in later in life. I think most 337 have the master as part of the side fuse panel (top right, if looking at it straight on). Our is a switch on the panel, just above the pilot right knee... a real PIA to get to without taking out the pilot seat.
I took a picture of the back of the fuse panel for later study... What I did see was clean, check some connections and hit it lightly with DeOxit. I want to mull over the service manual schematics, time permitting.

Tried seeing if we could pick up the same noise in flight on the ancient King 99 handheld. It was very difficult to tell in flight by just pressing the thing to your ear with the cockpit noise. I ordered the pigtail so you can plug a headset in.

I called my radio guy about the audio output caps. Of course, he told me he didn't think that was the problem...and he almost sounded like they didn't exist. I asked if he could just replace them, and I got the excuse that parts were hard to come by. With both the 12D and 12E making the same noise he didn't think it was radio induced. I left it at that as I've pestered him enough this week.

My next step is to track down the Narco radio ground on the pin-out...and find the common ground you're talking about. Trying to knock out the last of the obvious...so I've been raiding the internet for pin out diagrams. Access is to the back of the radio tray and chasing wires...I don't know how these avionics guys keep their sanity :-)