Doc,
The recording didn't do the noise justice in some regards. I was able to hear the noise coming through with music playing. I did what you've suggested and it seems that reinserting the tubes have helped. I haven't had that noise come back tonight at least. With any luck that did the trick!
The noise was pretty consistent, is it common for tubes to get stuck in oscillation? Also, is it normal for tubes to make small noises here and there even once warmed up? The crack is my first tube amp and I clearly have a lot to learn still!
Thanks for your help,
Paul