There are a number of fixes, but the symptom begs the question of what is causing the buzz?
I already know the answer, the Magnetek, univolt, or other oddly named battery charger. The best way to tell for sure is to turn off the univolt while on shore power and see if I am right.
There are filters that can be installed on the 12 VDC line going into the radio that will clean up the power. These can be had in most Automotive Audio departments of a Best Buy or Circuit City. If you have a local company that specializes in Car Audio they will have them too.
But back to the symptom. Has it always buzzed? Did it just start recently? Is it getting worse?
Have you checked the condition of the house batteries lately? Water level good? electrolyte level good?
If you have a dead or dry battery you have lost the best power filter you can have, the battery. Start there.
If it just started recently and seems to be getting worse you may need to plan on replacing the magnetk/univolt ASAP. Before it starts frying expensive stuff like refrigerator circuit boards and Furnace circuit boards.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato