Thank you, Albert, this was exactly right. I used a kitchen spatula to tap the tubes, holding it by the rubber blade and tapping with the wooden handle, volume way down. The first 6922 in line on the right channel (and none of the others) produced a musical "clink!" when tapped. Replaced it with a spare, hooked everything back together, and no more pops or hiss. Excellent advice--many thanks!
Pops and hiss from Sonic Frontiers gear
I have a Sonic Frontiers Line-3, retubed with NOS Siemens, coupled with a pair of Power-3s. When the preamp powers on, or switches between inputs, there is a loud, rough pop from the right speaker. This occurs not only when the power amps are on, but also when they're on standby. Also, when both pre and power amps are on (but not when either is on standby), there's a faint hiss from the right speaker at all times. I figure that the noises are due to a faulty tube but before I open the Line-3 up and start switching all those tubes around I thought I'd post this and see if anyone with similar gear has heard them. By the way, occasionally there's also a whistling sound from the right speaker, very high in pitch, but I think this comes from one of the tubes in the power amp, which are Svetlana Kt-88s. It lasts only a few seconds and tapping on the tube cage seems to lessen it.