When connecting the oxygen mask microphone U173/U to the microphone cord with a U172/U connector, are these pins polarized or can they be plugged in either way? It is just grounding the mic. wires. I can't see how they could be polarized. However in some airplanes when I plug into the airplane all is good and on some airplanes when I plug into the airplane squealing results. Me or airplane?? Am I looking at the wrong thing? Could it be airplane wiring?