Humming sound when using HDMI arc?

[Indydan]

I tried HDMI arc from my TV to my Wiim amp ultra. For some reason, it makes my subwoofer hum. The subwoofer hums even when the TV is switched off. Just plugging in the HDMI arc to the TV and Wiim amp ultra produces the hum. When I use optical instead, everything is fine.
I tried plugging everything in the same outlet. No change. I tried 3 different HDMI cables, no change. What could be causing this?

 

[HansD-NL]

You have (created) a ground loop. Maybe you have a cable box connected to the tv?
Using optical breaks the ground loop in your case.