Strange noises on some notes from a Laney TF200

[ZiggyZipgun]

I'm borrowing a small practice amp, and I've only been using the clean channel, but I noticed a slight electrical buzz when I play a B, B flat, or C.  To investigate, I kicked in the first drive channel, and it magnified it - definitely an electrical sizzling noise while that note is ringing out, and much more pronounced if I pick harder; once the note fades to a certain point, the noise suddenly stops.  It's a Laney TF200 Series 2 amp, which has an ECC83 (12AX7) in the preamp.  I'm not sure what voltage it runs at, and I don't know how much effect it even has on the sound, but the amp does sound pretty damn good for a little 1x12" combo. 

I know this isn't stompbox related, but has anyone run into this problem with this or another type of amp?

[petemoore]

but has anyone run into this problem with this or another type of amp?
  Yes.
  ~Could be the tube. Easy enough to try another.
  Could be something else...isolate the amplifier from vibration [set it ona chair or whatever...off the cabinet], at your own risk..bump the amp [don't beat it, just bump it a little from front/back and side].
  If it's not something like what those techniques would surface...how old is the amp ?
  And the answer to the next question would be: "Of the times I've had these kinds of 'wierd' sounds, the fix was either quick and easy or quickly went to difficult".
  Most of the times it's been a funky something like a speaker cable or something outside the amp, or a tube or loose tube socket. Bill had a speaker 'flex-wire' [between the tab and the cone] go intermittent once...took a while to even decide the cabinet was a problem...then pushing one cone in delicately there was a click from the other cone [4x12'' cabinet]...took a while to find that wire touching itself where it'd broken, the 'weld'...difficult also.