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The description of this sounds like worn or loose valve guides to me. There shouldn't be any significant amount of left/right (or radial, is probably a better term) play of the valve guide in its channel. One way to test this is to just take the valve guide out and work the valve without it there. If the noise you're hearing goes away, then the problem is with the valve guide (or the channel).
You can do similar sorts of tests by borrowing a felt from one valve and using it to double up on another. if the noise goes away with the doubled felt, that suggests maybe you're using the wrong thickness of felt.
But remember: Just getting the noise to go away isn't your complete goal. That result is only good if it's done in the context of the valve alignment being correct.
Diagnosis by internet can be difficult.
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As this problem is the 4th valve specifically, I would guess (difficult to diagnose remote) that the problem is the valve stem is slightly bent and the actual metal button is hitting the valve cap. Check to see if dead center and if not gentle push to bend the stem straight. The felt looks fine, while worn guides do not usually cause clanking.
If all else fails send video to dacapo@Wessex-tubas.com for advice from Chuck Nickles, Wessex designer and technical expert.
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I keep forgetting to update you guys about this. A few weeks ago, I finally got the nerve to ask Tuba Player Instrument Repair Guy about it, and he fixed it in under 5 minutes.
+2 to Mr. Hodgetts for guessing bent valve stem!
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That white plastic part is the guide. It has a little bump that fits into a hole on top of the valve, and is held in place by the stem.
I've seen where the stem gets loose when playing, and lets the guide rotate out of it's proper index.
Be careful, it can be broken if out of alignment.
Dennis