Several possibilities here. One is grounding of the MSD box; be sure the box has a clean chassis ground at the mounting point. You might also try a redundant ground wire from the MSD box to the engine block. At least try it temporarily with alligator clips and a length of braided ground strap.
Second is the 12 Volt power input line to your radio head unit or receiver might benefit from an RF choke (filter) in series.
What is probably happening is the oscillator in the MSD box is producing a signal that is rich in harmonics and strong enough to make itself heard in the audio path of your system. If the MSD box is metal enclosed (and they were many years ago), you should be able to eliminate the RF being produced in the MSD box by playing with different combinations of grounding or possibly moving the MSD farther away from the radio components.
Second is the 12 Volt power input line to your radio head unit or receiver might benefit from an RF choke (filter) in series.
What is probably happening is the oscillator in the MSD box is producing a signal that is rich in harmonics and strong enough to make itself heard in the audio path of your system. If the MSD box is metal enclosed (and they were many years ago), you should be able to eliminate the RF being produced in the MSD box by playing with different combinations of grounding or possibly moving the MSD farther away from the radio components.
