Magnetostriction is (almost) never the source of noise in an inductor or transformer. Airgaps - also in the form of poorly assembled cores - are a common source, loose windings is another. And yes, I heard once a shielding vibrate. 310-560 Capacitors sometimes make sounds. Quite unexpected. I heard it from a type-III ceramic one. But logical after all: it used BaTiO3 as a huge-K dielectric because it is ferromagnetic, and under polarisation it acts as a piezoelectric material and is commonly used in transducers. 310-502 Type-I ceramic don't do that, plastic capacitors neither. The get a hum from a 150kHz switching frequency, you must have something flawed in your circuit: typically an oscillating regulation - consistent with the bigger 310-345 capacitor stabilizing it. And by the way, 150kHz is already huge, I wouldn't go to 250kHz. The duty cycle can't be 100% nor 90% in an inductor buck circuit with 24V and 3.3V; and even with a transformer, 310-330 designers stop at <90% duty cycle so that the transistor always switches

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