RPM related AM radio interference?

[oldpurple]

I have a '79 with an AM radio. I get an annoying buzzing that is dependent on RPM. I can't remember exactly when it started, but I don't recall really changing anything where it started immediately after. I had changed plugs/wires/cap/rotor but I don't think it started right after that... I feel like it was just random. I have had it stored for the last 6 months though so my memory may be wrong.

Any ideas? My thoughts are ignition or alternator but I'm not quite sure where to start.