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Check the felts for both up and down you may have to change them. Try to put in the same thickness as when new or have a repair guy align your valves.

Either the felts are missing or compressed, or it's a worn valve guide. If the valve clanks when you twist it in the casing, it's the valve guide. The spring will cause the valve to try to turn as the spring compresses and decompresses, and if the valve guide allows it to do so it will clank when it hits the limit.
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Joe Baker, who is starting to get a little valve-guide "clank" in his tuba, too.

I recently ordered some new felts for my Yorkbrunner from Custom Music because I was having the same problem. They are made out of some sort of rubbery material, and they are great. They made the valves super quiet, and they have a bouncing action that I really like. The horn originally came with felts, and were never whisper quiet. Isn't technology amazing?

Do you guys have any magic tricks for a noisy valve spring? I've been told that it was most likely cause by how the spring was seated in the valve casing, but and every time since then I've made sure to have the spring centered after oiling the valve. It does help some, but the spring is still a little noisier than I think it should be.

Yamaha makes a piston valve spring that they coat with a thin layer of plastic that makes it very quiet. I use the springs in my Miraphone 1290. If that spring doesn't work you could most likely coat a spring yourself.
Good Luck, Phil