Electronic – Mechanical vibrations picked by an audio amplifier (Strange microphone efect when there is no microphone)

I opened today my Philips audio system (30W woofer + 20w satellites) because it has a lot of noise.
There is something weird with that amplifier: when I knock into the circuit board with my finger nail I hear in the sub-woofer a noise EXACTLY as when you knock with your finger into a microphone. To make it clear, doesn't sound as a component not being well soldered (one that randomly connects/disconnects when you touch it). The noise is very smooth and "high fidelity" (if i can call it like that) exactly as a microphone (connected directly on the mainboard).

I spoke into the circuit board (yes laugh if you want) but it won't pick my voice. However, the slightest touch of the circuit board will reproduce the sound into the woofer. So, it works only for very low frequencies.

I can upload a video (with audio track) if necessary.

I know that loose coils of inductors can create this effect but there is no single coil on that circuit board. What can create this effect?

Update:

It is not on the input. I shorcuted the input of the amplifier and knocked into the circuit board. The sound is still there. But the sound is soooo smooth, I cannot believe it could be a loose component.