You know I'm not sure I adjusted the picture control or not...that would be the simple answer I often miss
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The volume control does attenuate the buzzing. I also injected audio just before the volume control to see if the buzz was still present and it is not. Removing the tuner input to the 1st audio IF stage also kills the buzzing (and of course the audio).
Another thought that came to me in my sleep last night is changing the channel that the modulator is operating on. I'll give that one a shot if the picture control has no effect.