Sometimes a tube may have an internal short or bad connection. A cold solder joint to the tube socket could cause distortion/noise. Any of those problems could potentially be corrected by manipulating the tubes as you did, though it would be a temporary solution. If the problem recurs: try tapping each tube gently with a pencil. If it's a tube you can generally tell. Wiggling the tube within the socket will reveal a bad socket or connection there. If it never recurs: count your blessings. No- tubes will not have obvious distortion/noise during burn-in.
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