Popping sound

It is most likely a tube, but ut can also be a bad capacitor.   But before you take the covers off look around the house to see if anything like a plug in air freshener or anything that cycles on and off like that.  Pull them out while troubleshooting this type of problem.  

If you suspect the pre, swap the right and left channel tubes.  I believe with earlier cj preamps the channels each had their own tubes, not sharing a twin triode for both channels like more recent ones. 

Before switching tubes, try switching the speaker wires.

After that you can switch the interconnects from pre to amp

Good point about speaker wirr interference .   I had a ethernet cable running alongside a speaker wire and it caused a weird intermittent crackle noise.  

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).