Also consider using some contact cleaner on the tube pins and sockets. Think about it, on a tube amp with four output tubes two drivers, depending on the tube types, that is somewhere around 40 or more mechanical connections that need to be made. Corrosion or a bad connection on just one pin can create the noise you describe.
Sometimes switching tubes from left-to-right fixes the problem because the act of removing and reinserting tubes knocks off enough corrosion to address the issue.
Of course, tubes do age and/or go bad, so that could still be the issue. But the experiment above doesn't take much time and is completely free so is always worth a try.