> Hi - Thank you both for your replies.
>
> I am not sure what you mean by ''mechanical'' but, the sound is
> coming from the transformer itself. It is not too loud but I can
> hear it from a short distance away. The earth cables are all
> connected properly. My other 6U case is connected to the exact
> same power supply and makes no noise - this is what concerned me

Each transformer generates a little bit of mechanical hum. But if the hum of
the 9U case is clearly louder compared to the 6U case I'd recommend to
return the case if it's still within the return period. Replacing the power
supply (or the transformer of the supply) should solve the problem.

Best wishes
Dieter Doepfer