Update: I asked this question on Reddit and one person suggested the possibility that my coax going into my cable box might have a different ground potential than my home’s electrical system. They suggested putting everything back to where I had the buzz, then unplugging the coax from my cable box. Sure enough, the buzz disappeared! So now I’m going to look around my place for one of those surge protectors with coax connections in it. They said that might help.
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Matt