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Help - I want your non-loop knowledge

Seems this group has a diverse range of technical knowledge. Well I'm
having a slight problem my amp and thought I'd mine that vein. I hope
someone can give me a clue. My Crate VC3112 all tube amp (probably not
the rectifier) resonates with certain bass notes, making a raspy,
rattling kind of noise. The noise comes through the speaker and is not a
mechanical phenomenon (handle rattle, etc.) The $64,000 question is
this: is this the result of a microphonic tube? If so, how do I tell
which one. There are 4 12AX7's and 4 EL84's and none of them seem to be
glowing in a funny way. Any other ideas as to the cause?
  Thanks for your help.