Tube amps are never to be driven with no load on the outputs.
This has been my understanding too. It's always a good policy to shut it down, give it about 30 seconds to bleed off any residual charge, and then do your changes in wiring. Never run a signal through a tube amp without a connection to a load. Opposite of what Tarsando said, I've do not associate a similar limitation with SS amps (running a signal through an SS amp with no load connected will not likely result in any risk of damage - but I would NOT do any hot-swapping regardless). Since this has been my understanding, I hope anyone with contrary information might chime in. So, with SS I'd still power down before doing any wiring changes. I would not hot-swap with a live component, SS or tube. I've seen others do it with no consequence, but I think it's a bad habit to get into.
When you say you "unplugged the Rt channel only", do you mean you unplugged the interconnect from the amp/preamp, or do you mean you pulled the speaker cable connection?
It wouldn't hurt to replace the Chinese 6SL7's with some better NOS tubes from a reliable source. That would likely improve your performance, though it may not solve the problem. Not surprising that a pencil tap yields noise though...I should think that's pretty normal for an input/driver tube. That might be a good, and inexpensive start to troubleshooting the problem.
If you swapped valves, as you indicated in your post, with no change (noise is still in the right side/amp) it would seem to eliminate a bad valve. Your description of a "rustling sound" does, however, sound like a bad tube, so I still think either testing the tubes, or swapping them with tested tubes that you know are good would be a worthwhile approach.
Marco