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.