ARC Ref 5SE Pop sound when Mute is pressed

[TommyTunes]

When I press the Mute button and with the volume up and nothing playing, I hear a soft pop in the left channel. It just started making this sound, does this sound like a tube issue? I only have 680 hours on the tubes. If it's a tube any idea which tube?
Thanks

 

[DonH50]

Sounds more like either a coupling capacitor or bias/bleeder resistor open. I would call your dealer or ARC.