Ekon - There is nothing sacred about 612 kHz ( except to BT !)
It happens to be a convenient quiet spot on the MW band. BUT a RFI spectrum has nulls so it is just possible the RFI's null coincides with 612 kHz so try to find another quiet spot, possibly rotating the radio to null out a weak B/C station with the aerial.
Also you must appreciate that most things produce RFI at some level - long years ago I heard a story about static electricity caused by nurses changing at the shift end giving trouble in a hospital.