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Basically the lower you set the power of a strobe the softer the sound of the flash tube firing will be.
Any time you dim an incandescent bulb it will hum, or ring, or buzz a little by vibrating the filament so this is normal. That's all it is, is a dimmer that reduces AC voltage to reduce output of the light bulb. But if it's significantly loud there may be an issue with the dimming circuit causing the noise.