Here's a quick check for you. I've seen it twice, recently.

Check if you cable splitter at the outside of the house is grounded.

If not, ground it and it may solve the buzzing for you.

The 2 times I had noticed it, they were very strange problems, when lights are turned on, or when a sub's RCA is plugged in or when the Multiroom RCA feeds from the receiver were plugged in.

Anyway, maybe it will be that simple for you.

Jason Walter
www.electronics2you.biz