It sounds like you are describing switching noise. Usually, in well designed PSUs, it is inaudible. Sometimes, a loose component or transformer lamination will vibrate at the frequency used in the multivibrator circuit and provide a means for it to be heard. If the PSU is new, take if back and get a different one. It does not represent an issue that will cause it to fail, it's just a bother to have to listen to it.
Hoot