Earlier today I had an interesting problem - mixing products from MW stations all over HF. I actually noticed it a few times on the overnight SDR recording as well. I could not hear them on another radio, so that seemed to rule out them being "real", but instead being something produced specifically by the one radio.
After checking a few things out, I noticed one of the barrel connectors on the incoming coax, connecting two sections, was loose. I tightened it, and immediately the mixing products all vanished. The signal levels went up a little, as well. Perhaps something in the loose connection (dirt, etc) was acting like a rectifier?