AC hum in transmitted signal

[w8nsi]

I'd be suspicious of the power supply running the rig ... unlikely that an antenna change would create a hum on the transmitted signal. Sounds like a filtering issue in the PS, and the timing of the two events is likely a coincidence.

Another possibility is a ground loop. Good grounding is a must. Radio, amp, tuner, any auxiliary gear such as an adapter between computer and rig for psk31, etc. Clip on ferrites are handy for all long leads between equipment too.