Hopefully its something simple and/or external to your system. Tube starting to fail would be simplest and most easily resolved issue if issue is in system. If popping doesn't resolve itself via any of the methods suggested above I'd not continue using tube pre or amp, these things don't go away by themselves, continued popping could lead to much larger failure.
Popping sound
There’s an intermittent popping (electronic) sound coming from my left speaker only. It occurs at intervals of maybe 20 minutes or longer. I can live with it but I think it’s odd that it only emanates from one channel.
Any ideas?