06-05-2013, 03:39 PM
Selective hum like this is usually environmental. Try moving the radio to a different area of the house and see if it goes away. My workbench is insane for reception problems. There's some kind of field there that creates a ton of RFI, but if I move the radio under test off of the bench, even just a couple feet, all the noise goes away. Before I discovered this, it nearly drove me mad trying to figure out what was wrong with the radios. 
