All tubes are microphonic to a certain degree. That means if you tap them when in circuit you will hear that sound in your signal. Some are much worse than others, and can cause problems when they are modulated by vibration, or in extreme cases even by the sound you are recording!
How do you get a low-microphonic tube? Order one with that specification. It will have been tested, with many rejected from a batch. On average RØDE fail 20% of all tubes, this ensures that the ones we do use will be low-noise and low-microphonic.
For more information or questions about our RØDE products, please contact us here.