Arc Ref 3 tubes

I would think it is just mechanical noises of various components warming up and then cooling down. It should not be anything to worry about. Ever notice all the sounds your car makes after turning it off. Parts expand and contract with heating up and cooling down.

I suppose that could be. It is on a four season porch where it gets down to about 60 degrees during the night and 75 degrees during the day. I never play it when it's 60 degrees on the porch however. I guess if it was a bad tube I would hear it through the speakers?

Any chance it might be a relay? Does is sound the same as the MUTE relay? I have a LS26 and have had the Mute relays replaced under warranty and sad truth is they are little chinese relays. I had a REF3 as a loaner while mine was in the shop and they uses the same relays in both peices.