Had a similar problem years ago. Went all the way from CA to IN to CEDIA to take a class on troubleshooting audio buzz/hum in systems. Low and behold, it was taught by the owner of Jensen transformers. I learned great insight, not only as to the cause, but how to troubleshoot. I went back home and ordered some parts from Jensen (Ironically, about 40 miles from my home in CA). Between an audio isolator on an LFE sub input wire and another isolator on the coaxial input to the cablebox, it fixed the problem.

No common ground, slight difference in ground potential, and reference point looks like voltage, or an audio signal to the amplifier and it give you buzz/hum.