The other answer about magnetostriction is technically correct (the best kind) but Misses the actual cause, which is subharmonic oscillation. This occurs when you have not stabilized your control loop properly and is often the result of inadequate phase margin. A simple fix may be to allow the control bandwidth by increasing capacitance at the work amplifier output. But this may also make the response too slow.

For most people designing DCDC converters, this is the most difficult part to understand and correctly tune. If you get the parts selection right and carefully lay out the circuit, this is the one that they can't get right. It takes some understanding of control theory or careful testing and tweaking. And it's what drives a lot of folk to the expensive and relatively inflexible power modules.