What causes microphonics?
Loose electrodes, the metal pieces inside a tube which conduct electricity, are affected by vibrations generated by the loudspeaker. This causes the space between the electrodes to vary, and as a result the electrical fields to vary within the tube. This changes sound into current vibrations, which is positively fed back through the tube, and an annoying howl results. These vibrations can affect the electrodes through the socket or the base, or by sound waves striking the glass envelope of the tube. To help control microphonics Fender surrounds some tubes with a spongy, sound-absorbing material. In other amps and old radios mounting the sockets on rubber may also help. If both of these precautions fail the tube is too microphonic to be used, and should be replaced. You don’t necessarily have to throw it away, as it may work fine in an amplifier with a separate head and cabinet. REFERENCE: Marcus, Abraham. “The Electron Tube: Service Notes” Radio Servicing: Theory and Practice, Third Ed
