Noisy mobo ???

Is it true that a more powerful PSU will be noisier (not the exhaust fan) ?
or is it the sound of my AK31 mobo ?
It's hard to tell. Either it is the mobo that hums, or the PSU.
But I have a feeling that it is the mobo, because when PSU is turned on without a mobo, it barely make any noise.
Does it make sense that a mobo will make a humming noise ?

Humming would be caused by vibration. Make sure the motherboard is fastened securely. If that doesn't get rid of the noise try getting some dense foam and putting it between the back of the motherboard and the case.