Microphonic tube?

[MultiScaleMale]

I have a GK Fusion 800 and it just started making a sound similar to an airplane flying at altitude in the cabin or maybe more like the space in between stations on an FM radio dial, but lower pitched. Is that a microphonic tube? Dropping the amp a half an inch stopped it. I've been playing a long time, but mostly on solid state amps so I've never run into a microphonic tube before. Thx!

 

[seamonkey]

It doesn't read like a microphonic problem.

If it's under warranty - take it in.
If it's out of warranty the first thing any tech would do is make sure the tube is in tight.
If it get's to replacing things, they first thing a tech would do is change the tube.

 

[Killed_by_Death]

This is why it's good to have spare tubes, troubleshooting.
There are specific instructions for changing the tubes here on TB. I had to buy a specific nut driver to get mine out.
A plain 7mm ratchet won't fit.

The one I bought looks like this: