Well, I can't hear it but if it's deep in tone maybe AC hum maybe not? As it should or might also show up on XMIT? or not, something to consider it's tuff when you can't hear it in real time. I would suspect though that it's traceable you might start by making sure it isn't originating in the stages before the volume control you have a signal tracer use it to at the volume control with the tracer on high gain you may hear something if not use an audio dummy load or sufficient resistor to eliminate the speaker so you can turn the audio up full without a speaker this is to make sure it's inducing audio phonics or harmonics of any kind on the chassis. Just some things to try eliminating some possibilities. Unless you don't see it as something that is a problem for me it would be but I'm O-scar C-harlie D-elta.