Though anything may happen, the 300B are rarely the culprit of the symptom you describe. Same with the driver tube, not sensitive to microphony. It is usually the first stage input tube.
Never tap on tubes without knowing why. You stand a chance of causing a problem.
The simple way to determine the problem is to switch the input tubes left to right, since they are most likely the culprit. See if it swaps channels. Next do the driver, them power tubes.
If you plug this link into your browser you will see an explaination of microphony quoted from the tube "bible". The RCA Radiotron Designers Handbook 4th Edition.
http://www.upscaleaudio.com/rare/noisetest.htm
Never tap on tubes without knowing why. You stand a chance of causing a problem.
The simple way to determine the problem is to switch the input tubes left to right, since they are most likely the culprit. See if it swaps channels. Next do the driver, them power tubes.
If you plug this link into your browser you will see an explaination of microphony quoted from the tube "bible". The RCA Radiotron Designers Handbook 4th Edition.
http://www.upscaleaudio.com/rare/noisetest.htm