I would guess that the 12v dc you are using has some ac ripple in it from the converter. Your
boler probably has one of the older converters that do not filter the dc like my
Burro.
Try the radio from just the
battery alone and see if the buzz is gone.
If so you will have to put in some kind of filter. The
battery will act as a limited filter
Bear in mind that flourescent
lights will always cause a buzz especially on the am band, because they can radiate radio frequencies.