Chadnliz - this is fairly normal for a preamp. especially those with high gain. to some degree tubes are microphonic by the nature of their design. just like ice is cold by its nature. it shouldn't be a problem unless the music from your speakers creates a feedback loop through the preamp. otherwise just don't tap on the chassis while you are listening to tunes.
hal-O tube dampers may produce positive results in your system and they are inexpensive too. peronally i prefer a hint of microphonics since it adds to the airiness of the music.
hal-O tube dampers may produce positive results in your system and they are inexpensive too. peronally i prefer a hint of microphonics since it adds to the airiness of the music.