Hey GG&A, I've got a problem with my amp. It's a ValveKing 112

So I have band practice tomorrow and I was doing a sound check just to see if everything was in working order. I start playing a bit and depending on what note I fret I get this insanely annoying buzzing noise. I don't think it can be the frets because it only does it if I pluck the note normally whereas if I pluck it soft enough it's normal. Obviously, I'd rather have this fixed sooner rather than later.

I've noticed it's the worst on the fifth fret of both the A and G strings, but I just did every string at every fret and it's bad in some spots, and worse or ok at others.

Here's the kicker though, It's only on my distortion channel. Clean channel's totally ok. Is it bad power amp tubes maybe?

I think it may be the tubes because it's like a really fast rattling sound. So maybe it might be a filament or something. I don't know though. I'm a n00b at tubes.
It's probably a loose screw in the back of the amp. I have the VK 212 and had the same problem. In the back of the amp there are these "tanks" that have 4 screws in them, see if that's what's loose. If not check elsewhere. IF you don't have any luck finding the problem take it to your local music shop and have them take a look at it. Better they break it than you because they will pay to replace it. Best of luck.