Is my hum a ground loop?

[Uniquely]

I have an Onkyo 606 that is about a year and half old. I have developed a hum in the past month or so. It comes from all four surrounds, but is not noticable in the mains or the center. It is louder in the right surrounds. It is louder in matrixed options like PLIIX than it is in true multi-channel soundtracks like DD. It is non-existent when the receiver is not receiving an audio track, like when changing channels on the satellite receiver or in the start of a DVD when the warning screens come up.

I have not changed anything in the wiring situation... so there are no electrical wires any closer to the speaker wires than they were before the hum started. I do have one HDMI cable that is plugged into the back of the receiver that is not plugged into a source... but again that was the case before the hum started.

Does this sound more like a ground loop, or an issue with the receiver?