Wikipedia
Microphonism
Microphonism is the effect of unwanted modulation that is produced mechanically in an electronic circuit. It often may be alleviated by embedding the circuit in paraffin or silicone, or using some type of elastic suspension (rubber or caoutchouc are widely used).
When an oscillator is subjected to a mechanical action such as vibration or shock, its oscillation frequency changes and the result is a signal at the base oscillator frequency, frequency-modulated by the mechanical signal.
The usual test for microphonism in electron tubes is to tap the tube with a finger, light mallet, or small rubber ball while working as amplifier or oscillator.