Speakers hum/buzz when no sound is being played

[Automission]

Weird issue, anytime I have my TV on, or a Blu Ray on and I pause the video, I notice a faint humming/buzzing noise from my speakers, more noticeable if you put your ear close to them.

It seems to only affect speakers that are in use, i.e, If I change to stereo, only the front two buzz when no sound is being played, if I set it to all five, all five buzz. Also, if I hit mute on the receiver (even when the video playback is paused and no audio is being emitted) the buzzing sound stops.

anyone got a clue as to what might be causing this?

I've got the monitor audio bronze series connected up to an onkyo 606 receiver, using audyssey settings.

Is it normal, expected background noise, or could it damage the speakers in the long run?