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.
Next thing to do is to swap the tubes from one channel to another, one pair at a time. This will allow you to isolate whether is a tube and if so, which tube.
What Swampwalker said.
I will add that after you get your issue resolved (hopefully it's just a tube problem) that you purchase spare tubes to have for when these things happen.
Good luck.
Could be dirty pins on a signal tube.

Before you swap out the tubes from one channel to the other you might first try unplugging the tubes from the right channel one at a time and plugging them back in. Repeat the process a few times for each tube. This will clean the pins as well as the sockets.
Good advice from Jea48 as it would separate a bad tube from bad contacts.
You can also(sometimes) locate a noisy tube by lightly tapping those in the effected channel with a pencil's eraser.
Whatever you do to diagnose the problem, make sure you don't start swapping in different brands or types of tubes. If you do this you might enjoy hearing the difference that tubes can make and you start trying a whole bunch of cool new choices. This leads the slippery slope down the tube rollers slide.
In truth different tubes are fun but it can get addictive, and expensive.
Thanks for the very helpful comments.

I just got a chance to get back to the issue and started following the advice from Jea48. After pulling out one of the right-side power amp tubes, resetting it, and playing music through the amp, that tube seemed to have a clear visual difference...it seemed to have an orange glow inside that the other tubes didn't, as if the metal parts were heating up like in a light bulb. The distortion was also more quickly obvious. Just to be sure, I swapped it with the tube in the same position on the left side and the distortion went to the other speaker.

I've actually had the thing for less than 90 days, and haven't really physically moved it around at all since I first set it up. I guess the tube was just defective. Maybe Jolida will send me a replacement. But I can't wait around for snail mail to get my stereo working, so I guess I'll just go buy a set at store tomorrow. And maybe I'll try a different brand and see if that sounds any different. :-)