a strange noise now coming from my tube amp

Hi, I bought a Jolida FX-10 about 3-4 months ago and it has been great. But this week, while I was playing music, I suddenly heard a noise coming out of the right speaker...some brief crackling followed by a low-volume humming that sounded a bit like microphone feedback. The noise went away and then came back a few minutes later. So it's an intermittent thing, but happens often enough that I was able to do some testing and track it down to the amp itself.

To test, I swapped how the R/L speaker wires were hooked up, and when the noise happened, it was then in the left speaker. Then I swapped the R/L RCA inputs, and when the noise happened, it remained in the left speaker (which was at that point connected to the right-speaker output). So there's clearly a problem with the right-channel signal being generated by the amp.

I should add that the noise seems mostly independent of the audio signal level. Even after the power to the input source has been turned off, the noise will continue in the speaker for an indefinite period of time. Also, the noise doesn't appear until the amp has been on for about 10 minutes. Naturally, thought, the noise goes away as soon as I power off the amp.

I'm new to tube amps so I don't know where to go from here. There doesn't seem to be any visual indication that anything is wrong with any of the tubes. Obviously, I can try taking them out and putting them back in, but before going too far, I just wondered if these kinds of issues are common (and perhaps resolvable) with tube amps, or if I have something that I just need to send back under the warranty to have repaired.