sound from Pre Amp

I have Sonic Frontier SFL2 and Power 3(power amp) when the pre amp is still cold it sometimes makes audible noise at the speaker when I tab on the pre amp.And usually does not behave this way after it gets warm -hot.
i had a cj pre-amp that did the same thing when i tapped on it. i was told it was "tube microphonics" and nothing to worry about. just to be sure, call your dealer or sonic frontiers anyway.
Thanks Randalleron, I hope that's all there is.