Meaning: nothing to worry about. Some tubes will do this and its not an indication of failure or sound quality.
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I've seen solid state amps do this too. This is a bit more concerning than it is in a tube amplifier; it may mean that an active device is failing. If this is the case I think you'll find that it gets noisier to the point that you hear it when the amp is on- at which point it should be serviced, as the noise could be the result of an oscillation. I wouldn't worry about it other than that.