Funny sound from tube setup

Far as I know, the best way to find the offending member is to switch tubes between channels in pairs -- and when the sounds moves, that's your culprit. This, of course, is assuming that there's some audible sounds coming from a single tube in a given channel. If you're just wondering, generally, when to replace tubes -- there's no real answer or bright line to that one that I know of. Over time, the sound can degrade, and eventually go sour all together. Generally, it's a gradual process and one you might not even notice (like boiling a frog slowly, won't even notice until it's too late). If that's what you're wondering about, then the best bet is to suck it up and get a full replacement. Replace them, and if it sounds a lot better, it was time. If it doesn't, the put the old ones back in an rest assured that you'll need the replacements eventually....

The sliver v. black on the top of the tube is perhaps an indication of where to start, but not necessarily the answer. The silver is the "getter." (Layman alert, by the way). It's an oxygen-absorbing substance that they put in there to absorb any oxygen to enters the vaccume -- and when that happens it turns black. That doesn't mean that, if it's black, that your vaccume has been compromised on that tube -- in fact, I understand that's pretty unlikely -- as that can happen when applying the vaccume in the first place. But it's possible, so if you've got a noisy one, perhaps a place to start.

All that said, you sure it's a tube?