I think all coil based voltage transformers will emit noise. It is often due the coils attracting and repelling one another at each voltage cycle change.

Winding them tightly helps initially but the coils lengthen as they age and they loosen up. Varnish helps delay the loosening by binding the coils.

Is the hum mains frequency matched

That would indicate that the above case is likely.

I have only known solid state transformers not to hum.

i would mount the PSU boards on Rubber washers to minimise making the case a sound box.

Any concurrance

Regards Anoop

--- In Doepfer_a100@yahoogroups.com , "isoldeandisobelle" <isoldeisobelle@...> 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
>
> --- In Doepfer_a100@yahoogroups.com , Florian Anwander <fanwander@> wrote:
> >
> > Hello
> >
> > I assume you mean a mechanical hum. Right If so, then it should be
> > related to the method how the powersupply is fixed to the housing.
> >
> > Florian
> >
> >
> > On 07.07.13 00:31 , isoldeandisobelle wrote:
> > > Hi gang -
> > >
> > > I have a 6U case and a 9U case, the 9U PSU hums (which changes in frequency when different modules are in use.) - the 6U does not do this.... is this ok
> > >
> > >
> > >
> > > ------------------------------------
> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> >
>