Can a synchronous regulator "chirp" in burst mode?
I'll try and answer this myself and see what others think. The answer appears to be "it depends".

In general, a smps will not generate audible noise operating at their normal switching frequency (say, 500kHz). However, under light load conditions, a synchronous regulator may enter into a burst switching mode into order to reduce standby consumption (some can be prevented from doing this). When the frequency of the switching bundles enters the range of human hearing, audible noise may be generated:



The amount of noise emitted can be mitigated using careful design. Transformers/inductors produce electromagnetic fields which cause forces between the coil elements, setting up vibrations in the component. This can be minimised by varnishing or using adhesive. Ceramic capacitors exhibit piezoelectric properties which depend on the types of materials used in their manufacture (barium-titanate is particularly bad). Using a Class 1 capacitor or a film type instead of ceramic type can reduce audible noise.

Other than that, some manufacturers may have IC specific advice in their datasheets. For example, the LT8610 can use Burst mode or pulse skipping mode depending on the logic on the sync pin (I don't know what effect that would have though!).