The real way is to find the problem inside the TV set (an IF tube getting cut off at the high white parts of the video (which makes the channel carrier RF to be at its minimum), causing "holes" in the sound IF carrier, thus the buzz, as the video repeats at the vertical rate), but the lazy way is to change the video amplitude inside the converter box. :D Here I'm using a Zenith CECB. What I did was to take it apart, to get at the bottom of the main board, under the metal box that holds the ATSC receiver, and the RF NTSC TV modulator. This box has two sets of terminals in a single line. The terminal that has the baseband video that feeds the modulator is the terminal closest to the middle gap, part of the 5 terminal grouping. I used a 75 ohm resistor to ground to add extra loading on the video signal, thus reducing the video amplitude the modulator sees. Now the max white in the video causes the channel carrier to not be so reduced. Thus the bad IF stage no longer cuts off the sound subcarrier.

It would be easy enough to restore the CECB to stock, just remove the resistor I placed under the board.
http://www.videokarma.org/attachment.php?attachmentid=177310&stc=1&d=1356051609