That's the most compelling argument I've seen to explain this...when Ron
mentioned that the modulation was in the Q channel, you had to know that
it wasn't coming from conventional amplitude modulation.

Here's my recording of the "WFAN hum", as received today on a
battery-powered Sony SRF-A100 receiver in wideband AM Stereo mode:

ftp://ftp.amstereoradio.com/uploads/wfan-hum.mp3

This was received in C-Quam-compatible mode, but switching to ISB
(Kahn) mode yielded no audible difference. A spectrum analysis of the
L-R component reveals it to be a 120 Hz hum with multiple harmonics
extending well above 1 kHz... so it's more of a "dirty" hum, akin to
that of an audio device with inadequate filtering of its DC power
supply.