Hi there Dieter, thank you for the reply.

More than anything, I think that it may be outputting the same amount of hum as the 6U, but appears louder due to it's larger size and emptiness (soon to be remedied! :) )

I have learned something about transformers and power supply by asking this question - although an essential/vital component, I had never really gave it much thought beforehand.

Thank you again and best wishes.

--- In Doepfer_a100@yahoogroups.com , <yahoo@...> wrote:
>
> > 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
>