I am having an issue with my audio panel and only through Com 2. Instrument list: GMA 345, GTX 345, Garmin 530W, Garmin GNC 300XL, G5 HSI, JPI EDM 900. There is a recurring noise through the Com 2 on the audio panel. It is the one the GNC300 goes through. Volume does not affect it. I have changed out the GNC 300XL for another one and nothing changed. I pull all circuit breakers except for the GNC 300XL and the audio panel and the noise is still there. It happens whether the engine is running or not. I can be sitting in my hangar with just the master switch and avionics master on and nothing else except for the audio panel and GNC 300XL and the noise is there. I have checked the antenna, the coax, my avionics shop is stumped and they put in an inline noise suppressor and still no change. They have checked all of the grounds and shielding. I am left with the possibility of the audio panel itself being the problem. Even when I fly and am away from the airport this noise continues so I don't believe it is anything at the airport. And of course, when you turn off the GNC300XL and there is no input to the audio panel the noise goes away so it is not just something coming from the audio panel but maybe the audio panel is having an issue when sound is coming through the #2 Com. I have attached a link to the audio noise. It is actually a video on Youtube but it is just for the audio so I could send it out to see if I can get any help. Right now my avionics shop is stumped and so am I.