Sounds like tube microphonics. There are several tube dampers on the mkt. Do a search or google them. It might also be a funky tube that needs replacement. Microphonics is one of the downsides of tubes.
Tube Amp "Ringing"
I have a Jolida SJ-502A tube intergrated amp and tonight I noticed a "ringing" sound coming from the amp. Didn't notice it when the music is playing, but when I changed my CDs and was close to the amp, noticed the ringing. Was curious on what it could be and how I can solve this problem. btw, I'm a "tube newbie", and help and suggestions would be appreciated. Thanks.