Hi Telemetric,
if you're talking about the EA-70(S) AC adapter, it's
absolutely normal.
These units start to humm a bit when the Zaurus is
charged or if no device is connected. Fortunately
we have such a noisy office, that we don't notice
this usually. But this behavior is reported from time
to time in all user forums since Sharp started selling
the Zaurus...
///TRIsoft
Marc Stephan