It's almost surely a switching power supply. If you can run the rig off a battery that would eliminate anything else that might have been powered by the genset. First thing I would try after that is to see if I could pick this up on a handheld AM receiver you can DF with. Lowering the frequency might allow picking it up with a smaller antenna. From there (generally) the higher in frequency you pick up the source the closer you are to it. I've started with the interference picked up on 80M and 25 feet away I was hearing it on 2M. Noting when you do and don't hear the noise can be a clue too (24/7, day/night, hot/cold, dry/wet, etc).
Mark K5LXP
Albuquerque, NM