I would not be concerned about little noises as the tubes warm up from being cold. But if it is constant after they are hot, try swapping the tubes around. It could be that the bias is way off. If the same tube is noisy, you should return the tube to where you got it and they should replace it. This is not normal. If they were a matched quad, they should give you a new set. You should also be careful not to burn your ear when it is close to the tubes.
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