Tube burn in. I had the exact same issue. Pull the back tube out, turn the amp on and let it sit for a few days with just the front tube burning in. Mine needed a total of 5 days straight to get the noise out. It worked though.
I’ll see if I can find the article where it explains this and link it. My memory is faulty sometimes, I want to make sure I pulled the back tube out though, I can’t remember clearly…
Go read through this thread. It has a TON of useful info for you.