I am not too sure of what this is, but it sounds like a Diesel engine with a pulse rade of 45 per second. Every 3 or 4 minutes it will pause then have a buzz then another pause then go on with the regular 45 per second pulse. I heard this on 8022 kHz to 8083 kHz which is tha maratime band area. I heard the same sound on the bottom of 40 meters the next morning ( around 07:00 CST ) it was weak, but it covered 6970 to 7007 or better. I have some sound clips, but no way to post them.
Any ideas?
Thanks
Herb KC5HMC