I hear it too. Over the air HD on the Global Canada feed.
Maybe a poorly set automatic feedback suppressor?
It sounds to me like when you have a telephone hybrid with a mix minus problem. Causing ringing from the voice coming back to you over a telephone line or cell phone. I know that isn't it, but it sounds exactly the same.
"...actually I can hold a note forever. But eventually that's just noise. It's the change we're listening for. The note coming after, and the one after that. That's what makes it music."