I remember two distinct signals, one was rhythmic chatter and the other an annoying buzz-saw. I was told the later was Russki jamming.
We used to hear jamming (signals meant to keep frequencies clear from international use - Cuba did it a lot)....we called them bag pipes.
They did this to keep those frequencies clear because at some point during the day or night they planned on using them and wanted to keep them clear. For long haul HF, changing frequencies throughout the day was a rule. Higher freqs in the day and lower and lower ones at night. Those D,E, F1 and F2 (these two mostly) layers of the ionosphere are fickle.