Although the sound probably originates with a tube in your preamp, often there is nothing wrong with the tube, just the contacts with the pins and socket, and the problem can go away by just wiggling the tube or taking it out and reinserting it. Reversing the tubes can also solve or identify the problem (if it's a bad tube).
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?