Finally got to removing the alternator. The humming sound ended when disconnecting the multicontact. Check with voltmeter in contact gave as follow:
P = Not used
L = 4,6 mV
F = 1,5 mV
S = ~ around 100mV
Ignition off, battery connected.
I initially belived that someone had mounted the wrong alternator but I don´t know enuff about the alternator to understand what´s wrong. Please help.