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Good job so far. The cap by the cowl flap motor is likely for the motor. The cowl flap motors put a lot of DC hash on the line while they are operating. If they make noise in the headset while operating you may have luck central point grounding the ground lead. The flap system ground is located near the switch on the panel. I was able to completely eliminate headset noise from a flap system by lifting the local ground and moving it to a central ground bus. You can pull the capacitor in question and measure it. Foil wrapped caps can dry out and go bad. Unfortunately there is no printed spec for the value of these caps.
Back to the radio noise. My favorite antenna cable is RG400. That would be the ideal for all your antennas. You could try reaching up and disconnecting the antenna while in flight. Do no transmit on your radio with the antenna disconnected. See if this changes the receive static. Let's also disconnect one antenna and see how the other radio sounds.
I'm assuming you have a good volt meter. While we are at it we should check some voltages. Clip your test lead on the avionics bus and then pick a good ground for the other lead and go flying. You are looking for 28.4-28.8 volts. The next thing to do is set the meter to millivolts AC. This will read AC hash on the line. You will to see that between 50-150 millivolts AC. The reason you are seeing AC on the line is that the rectifier in the alternator is not 100% efficient. A bad diode in the alternator will cause you to lose a phase of the three phase alternator. The alternator will current limit. It will make proper voltage until you put a load on it. This can induce all sorts of interference on the line directly into your radios.
While you are a it lets listen to the VOR audio. It would be nice if you had a VOR station that broadcasted HIWAS. This would help separate out Nav from comm or tell us that the problem is affecting the entire radio. This test would also shed some light on the antenna circuit for the comm radio.
Report your findings.
Dave
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