When looking at inductor circuit it is common that you may hear an audible noise. It is often hard to trace this noise and identify where it is coming from. When there is magnetic energy stored in the inductor air-gap, it can vibrate and generate noise. This noise will come from the inductor air-gap materials and magnetic circuit geometry. Depending on the load condition, frequency and the part variations, the amount of audible noise will vary.
So how can this audible noise be reduced?
Apply a liquid varnish or RTV Silicone to coil if the inductor is soldered on board.
If there are many inductors on the circuit board, you could consider using another version of inductor or having the core and winding vacuum varnished, which can reduce this noise.
If it is possible to change the switching frequency in the application, try to adjust the switching frequency above the audible range (20Hz - 20kHz) to reduce this noise.