ePMP Power Supplies Quietly Buzzing / Hissing

I've found some of my ePMP power supplies have a quiet buzzing / hissing sound. Have others found this issue? Of the three on my desk, only one is doing it, but I've noticed it with others.

It is just loud enough for me to go looking for a noise. 

It is normal. Usually it means a loose transformer core / windings, or some ressonance going on in the filter capacitors. 

It can be annoying, but i've seen power supplies go on like that for years.

Unless they are making really loud noises, like a screaming beep, don't worry about it. 

the ".......tick-hsssssss-tick..." is normal for this kind of PSU.

More info about this noise:


I completly agree with Guilherme: it can be annoying, but it doesn't mean the PSU is defective.