When transmitting, stations receiving my transmission report what they describe as feedback in my audio. I had a station record me, and indeed it does sound like I have a receiver on in the back ground on the same freq, but this not the case. I performed a hard restart, and didn't cure the problem. Radio is a 6400 and using a maestro,running version 2.39. I also tried transmitting on my i-phone and had the same problem.