Rattling Amp

A microphonic tube is a common source of the problem you describe. That means some of the internal parts of the tube are loose and can vibrate. Every tube is a little microphonic, but some are shaky enough to cause a problem. The parts are electrically charged, and when they vibrate act like a condenser microphone. Try gently tapping the tubes with something like a pencil with the amp on and see if it produces that sound. If so, replace the offending tube. And be very careful not to drop or bump your amp, as tubes are fragile and mechanical shocks can easily make them microphonic.