A strange “snap“ or “popping” sound keeps interrupting our music

Turn your dac off and unplug it. Then take out the tube(s), use contact cleaner and clean the tube pins. Reinstall the tube. See if the problem goes away (unlikely to but at least you covered this base). This issue could also be caused by a bad/loose connection. Check all interconnects.

is this popping heard thru both channels?

Mainly the right channel.  And not consistent.  I’ve wondered if the weather has an impact.

pS, I will try your home remedy this evening, and thank you.

Hook your TV or some other analog source up (or swap out DACs if you have a half dozen of them laying around like some of us do) to get your DAC out of the path and see if the problem goes away.

Jerry

I had popping noise immediately when I had my new DAC. All things being the same, one would assume it was the dac's fault. But I switched to a better USB cable and the popping noise was gone. 

For clarification, the popping noise with the usb cable didn't have popping noise with a previous DAC.