Find all the hams in the area and see who can hear it.
Those who have beams can try to get a heading on it. You
may find that 6m beams give the best headings when they
can hear it.
One thing you need to determine is whether it is a
distributed source (like a bug zapper that many people
have in their back yard) or a single source (an industrial
machine running on 13.56 MHz, for example.) If all the
bearings point in about the same direction, that is a
good sign that there is a single source.
Then get or build a DF antenna and track it down. DF
antennas for CB are not uncommon, and may be a good start.
(Use AM or SSB rather than FM receivers.) DF loops have
a lower sensitivity than beams, and can get confused by
multiple sources, but may be the only practical method
for taking bearings on the lower bands. If you get
close enough that you can hear it on 2m or on an aircraft
receiver (120 MHz), then yagis are practical. You may
even be able to track it with a TV antenna if it is
strong enough to cause interference. Here is a good
site for building cheap beams:http://www.clarc.org/Articles/uhf.htm
You might also get some useful information from K0OV's
HomingIn web page about DF:http://www.HomingIn.com
You might also see if there are any hams in your area
who are active as transmitter hunters - they are used
to tracking down signals quickly over long distances.
If you need more specific details on DF equipment, let
me know and I'll see what I can go.
- Dale WB6BYU