[SAI_Peregrinus]

If you want extra tinfoil hattiness, most multilayer ceramic capacitors (the surface mount kind that are everywhere on PCBs) are somewhat microphonic. Condenser microphones, especially electrets are basically just capacitors designed in a special way to maximize this effect.

[drdaeman]

Interesting! I know about condenser mics, but I thought a typical SMDs are way too small to sense anything useful. Had anyone experimented with this?

[SAI_Peregrinus]

They generally are too small to be useful. That's why I called it "tinfoil hatty". Most of the issues with them come from mechanically coupled vibrations in a system (from fans and the like) flexing the whole circuit board, so it's not as big an issue with phones. Also most of the capacitors are power supply decoupling caps, so difficult to sense variations from.

It's mostly just a fiendishly annoying effect in high-sensitivity test equipment.