Topband: Help - Mysterious Noise on 160 meters

[Tod-ID]

I would appreciate the assistance of readers in determining the generator of
the noise signals I am observing here in Idaho on 160 meters using a pair of
K9AY loops. The loops perform as expected on signals from various sources
and as I switch between the four loop directions it is reasonably evident
which direction the amateur signals are coming from. However, the noise that
I am observing does not appear to come from any particular direction that I
can determine. 

The noise is in the form of spikes or pulses that are separated by exactly 4
kHz. They are about 25 dB above the noise floor of the K9AY loops so they
are very noticeable. As one tunes across the 160 meter band there is a
series of tones and on a spectrum analyzer the display is similar to the one
that is produced by a comb generator. The amplitude of the signals drops
slowly from 1700 to 3500 kHz. There is a slight variation of the pulse peaks
- sort of a slow ripple. Every 90 seconds the signal on a particular
frequency stops for about 2 seconds and then resumes. The signals are
present 24 hours per day.

Located here in Eastern Idaho I am not surrounded by many amateurs who are
active on 160 meters -- in fact I may be the only one for 50 to 75 miles
from my location -- maybe further. This makes it a challenge to see if
anyone else is noticing the new signals. The signals were not present when I
was listening on 160 meters last year  --- or at least the RX antennas I was
using did not receive them at a signal to noise ratio that made them
noticed. 

I invite people who have some experience in identifying "unusual signals" to
look at the spectrum scan pictures on my web site and send me their ideas.
With some idea of what might be producing the signals I might be able to
locate the source more rapidly.

The link to that part of my web site is:

http://www.k0to.us/HAM/Noise/4kHz%20noise/index.htm

Thanks for any help you might be able to provide,

Tod, K0TO