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That buzz can happen in intercarrier sets. The sets that get the 4.5MHz sound IF off the video detector. If the video IF is not biased right, the whites in the video (which occur at minimum RF modulation) can result in no video RF carrier, and thus the sound carrier aso disapears momentarily. Which in turn results in a buzz at the vertical frequency, 59.94Hz (this because the pattern of white lettering repeats at this rate, the video field rate).
When the analog NTSC transmission system was designed, they decided to make the tip of the sync pulses be the max RF modulation, and peak white the minimum, but still 10% that of the sync tip. If the IF amps shave off that bottom 10 or more %, you will get buzz in intercarrier sets. Making the sync tip be the max allows the use of keyed AGC, as that part of the video signal is always a known reference, and has lower noise issues.
So I'd look at the IF strip for weak tubes or bad biasing, or bad B+. Badly aligned IFs could do it, but I would avoid messing with the alignment, as it's harder to get right than that of a radio.
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