Unusual hiss issue - Searched and not found!

I have an unmodified Jolida 1501RC integrated, and recently I've got a new issue: when I power the amplifier on, there is about 10 seconds of loud hissing that slowly fades to very low. It's still audible (ugh) but it's much lower than it is initially.

So far my reading on this topic has indicated that it could be a gain issue, a tube issue, and I've also seen folks talk about the issue not being with the amplifier at all, although that seems unlikely to me.

Any ideas, guys?
Replace the output tubes. Easy fix.

Matt M
I agree with Matt up to point. It sure sounds like a tube to me. But which one?? Is it in both channels? If so sounds like an input tube. But if both channels it could be driver/splitter tubes or an output tube. If in just one channel reverse the right and left channel tubes.

If you are not familiar with the Jolida 1501RC, you should at least take a look at it before responding. It is a hybrid integrated and has no output tubes or driver/splitters. It only has 2 preamp tubes, and if noise is just one channel, then try swapping those as suggested.

My bad - When I think of Jolida I think tubes. A hiss that fades is a typical problem with tubes and or maybe a socket.