[Lowfer] Strange QRM here on SAQ frequency

[Bill de Carle]

Hi Everyone:

Anyone on this reflector know what could cause some very strong QRM I 
see right on top of SAQ's 17.2 Khz frequency?  It consists of a group 
of spectral lines spaced 120-Hz apart, centered near 17.2 Khz but 
occupying only about 1 KHz bandwidth.  This QRM is present most of 
the time at my QTH but oddly enough, it wasn't there on Sunday 
morning when I got up around 4:30 am Eastern time.  Obviously it's 
powerline-related, but as far as I know (famous last words!) it 
doesn't seem to be coming from anything in my home.  I can even 
disconnect the mains & run on batteries: the signal does not change, 
so it is clearly radiated, not conducted through the power line.  The 
amplitudes of the individual spectral lines vary slowly (cycling over 
perhaps 20 seconds), seemingly at random.  I live in a rural 
neighborhood and there are some electric fences around.  What bugs me 
is why at 17.2 Khz?  Have you seen this signature before?  Here's a 
screenshot showing what it looks like:

http://www.magma.ca/~ve2iq/saq_qrm.gif

Any ideas would be much appreciated.
73,
Bill VE2IQ