I agree with Andy... Most of the circuitry that can cause a buzz should be in that IF pack. However, I think I recall some of the audio IF being on the small audio module, which has 2 or 3 adjustable transformers on it. I never had to fool with those, as I just replaced the module if there was trouble.
I am going to guess that you're hearing video leaking into the audio, possibly a VIR signal (same thing that causes the hated retrace lines at the top of the picture on some channels that use the VIR bandwidth).
So the IF and audio modules would be the first things to try. I probably have those modules, so if you can't find one locally you can eMail me at
[email protected] (I seldom check AK messages, so eMail is always best)
Charles