Hi Felipe,
the EA-70 charger starts to buzz if it's in standby.
During the charging process, it should be silent.
This is a known thing since the first SL-5500 series
with a ton of threads around ("help, my ac adapter
humms after it finished charging my Zaurus...").
So, i wouldn't worry about it (at least, if it's a
relatively quiet humming sound). In our office,
we have to place an ear directly on the charger
to hear it
///TRIsoft
Marc Stephan