Greetings:

Seeking opinions on an old, commonly heard signal (AM) on HF, of some
considerable power, with a very memorable sound.

This will be a bit difficult to describe and I wonder if some folks
would have audio recordings of spectrum surveillance from the 1960s
for an actual sample, but in lieu of that I will try to convey a
sense of what it was. I had always assumed that it was photo
transmissions by the wire services, but listening to modern equivalents
I have doubts; it actually sounds more like the early telephone
voice scrambling systems that were acoustically coupled to the handset,
and which produced a continuous noise irrespective of voice level.
The sound on the air was much like hearing several P51 fighters
approaching at high airspeed and props not in phase; the spectral
power is mostly in the range of about 200 Hz to 500 Hz with a
varying heterodyne of several Hz. It had a bandwidth of at least
25 kHz (can't say more precisely due to the cheap receivers I had at
the time). I don't remember the frequencies, but at the time I would
have been concentrating on monitoring 3 Mhz to about 12 Mhz, and these
signals were strong at all hours on perhaps a dozen different frequencies.
I always regarded them as annoying QRM.

I did little SWLing from the mid '70s until somewhat recently, and of
course there is nothing like this heard nowadays.

At the time, my QTH was about 25 miles from the largest National Guard
training camp in the midwest and there were four USAF bases within 200 miles
as well, so it could well have been some modulation mode used by the
military.

I would really enjoy knowing if others remember a signal like this
and knew its origins.

Regards,

Michael