Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660,
does anybody else?

I've been hearing a background hum on WFAN for a LONG time, but only
when listening in "forced" AM Stereo. When listening in mono, the hum
disappears.

Normally, AM Stereo receivers rely upon a 25 Hz "pilot tone" that is
transmitted by an AM Stereo station, which tells the receiver to
switch into stereo mode. However, unlike FM Stereo, this pilot tone
is NOT necessary to receive AM Stereo, so some receivers --
particularly from Sony -- operate in "forced" AM Stereo mode, which
receives _all_ AM signals in stereo mode, regardless if a 25 Hz pilot
tone is received or not.

Thus, with my Sony receiver, I can listen to WFAN (and other "mono"
stations) in "forced" AM Stereo mode, and in this mode, I hear the 120
Hz background hum. However, the hum is "out-of-phase", indicating that
it is being received in the L-R "stereo difference" component of the
signal. When the receiver is switched to mono, the L-R component is
not received, and WFAN's hum disappears.

I once asked WFAN's engineer about this hum, and they said WFAN was
not transmitting it, and that it was being caused by my reception
conditions, such as electrical interference. However, I hear this hum
from WFAN, and ONLY from WFAN, 24 hours a day, even when there's a
local power failure and thus no electrical interference can possibly
be generated!

WFAN _was_ an AM Stereo station until about two years ago, when they
reverted to mono. But, it IS possible that their C-Quam AM Stereo
exciter is still installed and operational, but merely is being fed
with mono audio and has its 25 Hz pilot tone generator switched off --
which, for all intensive purposes, means WFAN is transmitting a
conventional "mono" signal. However, if that AM Stereo exciter ITSELF
is the causing this 120 Hz hum, it could be introducing it into WFAN's
signal in the manner that I described -- only "forced" AM Stereo
listeners will be able to hear it, while mono listeners won't hear the
hum.

Also, listeners with sophisticated communications receivers may also
hear it in Independent Sideband (ISB) mode, which operates in a
similar manner to AM Stereo reception and can pick up the stereo
difference component transmitted by AM Stereo stations (although not
in a manner which provides correct left-to-right stereo separation in
the received audio). Thus, with this kind of receiver, you may also
be able to hear this mysterious "WFAN hum".