What might cause audible noise in an inductive charging system?
Wireless charging pads work by inductive coupling. In the pad is a coil, and in the device being charged is another coil. When these coils are close together, they have a high mutual inductance, and we can use this mutual inductance to transfer energy between them, as in a transformer.







simulate this circuit &ndash; Schematic created using CircuitLab

The trouble is this: an inductor is also an electromagnet. As the current oscillates in the coil, the magnetic forces also oscillate. These magnetic forces act on the individual turns of the coil itself and ferromagnetic materials nearby, causing audible vibration. Magnetostriction can also play a role.

Besides the charging coils themselves, there are probably more inductors in the circuitry that converts the 60 or 50 Hz mains AC to the higher frequency shown here.

I don't know that there's much you can do to mitigate this noise, other than remanufacturing the device. You might try setting the charger on a neoprene or rubber pad, which might at least prevent some of the vibration from coupling into your table. The better solution is usually to move the oscillation frequency above 20 kHz where it can't be heard by humans, or to more solidly support the coils so they can move less.