Bruce,I may have your solution. This could be a 2nd-order IMD product from AM broadcast stations. For example, you have a very strong signal from WRKO on 680 kHz in your area. There would also be a reasonably strong signal from WEEI-AM on 850 kHz (the old WHDH, from back when I had hair). 850-680 = 170 kHz. That seems the most obvious combination, but there could be others. The late Bill Ashlock, not a ham, but an avid LF experimenter who lived in Andover, used to have receiving problems because of the strong WRKO signal.
What you could do for detective work depends on your LF antenna situation. If you are using a whip and preamp (such as a MiniWhip), you could try a high L, low C series tuned trap for 680 kHz across the input. If your antenna system doesn't use a hi-Z preamp such as that, a parallel-tuned (low L, high C) in series with the feed to your receiver or low-Z preamp would be best. If you give me some info on your antenna and receiver setup, maybe I can be more specific.
In my case, I tend to use loop antennas because of a very strong signal on 580 kHz that has defeated all my experiments with untuned whips. I am able to run a 50 Ohm system, and use a low-pass filter ahead of a preamp to get rid of most of the AM broadcast signals.
John, W1TAG