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It might be the RF modulator doing it. You can lower the video signal level feeding the modulator by adding say 75 ohms to ground. Or lower resistances. The buzz happens when the picture carrier gets completely cutoff, white level is the minimum RF modulation level, sync tip is max RF carrier level. Get too white and the picture carrier disappears, and the intercarrier system no longer sees a 4.5MHz sound IF. "Holes" in the sound, yields the buzz.
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