Does your TV have the option to try different sound settings? Eg 5.1 emulation, virtual surround sound, etc?
Given that different content is producing different results, I would start there.
It's possible that on demand audio is encoded differently than live audio, and the way your TV sound is currently set is doing weird things to the live audio.
Interesting. I get this, too, and find it very annoying. I have to turn the volume down to tolerate it. I was believing the problem was my hearing aids, but I’m going to pay more attention now to other sources. I don’t believe I ever hear it with my AppleTVs that are connected to my LG sets. I suspect I just hear it when viewing Optik TV through the Telus box. I have several TVs (LG and Samsung) with AppleTV connected to all of them. I’m going to pay attention now to where I hear this most. I’m pretty sure News channels are the worst.