Sometimes tubes will develop a bad internal connection that improves as the tube heats up. I've had some that would make that noise unitl I tapped them with a pencil. They never seemed to get much worse, and since they were my hardest to find tubes(Sylvania 6SN7Ws & 7199s), I put up with it. A connection that opens within a tube shouldn't cause any further problems. An internal short can cause a plethora of headaches.
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