Being new to tube equipment myself, I've researched this issue. It seems that there is quite a bit of variablity from tube to tube as far as microphonics and noise. Especially among certain nos tubes. It would be woth your time to go to www.vintagetubeservices.com and read what Andy has to say about these matters. He tests his tubes for these problem areas and grades them accordingly.
Microphonic tubes...
I have noticed that two of the 4 6GK5's in my CJ Premier 10 preamp are somewhat microphonic, producing an occasional high freq whine. I can tell by lightly tapping them lightly with my fingernail. One recently went bad, so I replaced it with a spare I had laying around. System sounds ok - no whine anymore, but I think the tubes are rather old, and I am going to go ahead an replace them all, how do I go about selecting a non microphonic tube with good sound. I can get a set of 5 NOS tubes of either GE or Mullard for cheap - $15.00. Any inputs, other suggestions on which tube might be better from any tube masters out there would be very much appreciated...Tim