It most often happens when the frequency of the channel of interest coincides with the center frequency configured on the dongle. If this is your case, then shift the centerfreq a bit up or down (say, 100 kHz) and the hum should go away.
I believe there is something wrong with the current setup of YPPH feeds. Departure feed serves the audio of Tower, while Ground is in fact Approach. Are all these feeds yours?
Are you using RTLSDR-Airband by chance? Can you could post your config file (without passwords, etc)?