OPs troubleshooting technique was solid; any time you mix high gain with something non-linear with radio transmitters nearby, you can assume weird audio range w

OPs troubleshooting technique was solid; any time you mix high gain with something non-linear with radio transmitters nearby, you can assume weird audio range waveforms on the O-scope are just the local sports-talk AM radio broadcast station showing up in what boils down to an overgrown crystal radio.