#1
I have a Peavey Valveking 212 that makes a weird garbling noise on the clean channel. It kinda sounds like some weird space/time warping. The volume knob on the amp does not make it any louder or quieter. It makes the sound if a guitar is plugged in or not. Oddly enough, it goes away when I turn it onto the gain channel.

So I think it's a preamp tube. I tried the tapping trick and didn't seem to get hard evidence, so I'm pretty unsure.
#2
I own the VK112. The tapping trick may not work in this case. It is usually used to locate microphonic tubes. I'll give you bit of free advice, as you own a tube amp. Ideally, you really want to have a spare set of tubes sitting around and ready for troubleshooting problems like this one. That helps to make it easier for troubleshooting the occasional problem - simply swap in a known good tube and find the faulty tube.

Another troubleshooting trick is one of the tubes inside your amp for troubleshooting. Quite often, the effects loop will have its own tube, which isn't used if nothing is plugged into that circuit. Remove that tube and use it to swap into each position, until you find the bad one.