surely you need to press the brake pedal too in order for the pump to activate?
are you certian the pump is running coz the throttle body ilde controller can make similar sounds that can make you think the pump is running.
really you need to get the car diagnosed by software capable of communicating with the selespeed ecu.
pressure range should be 45bar to 55 bar.
on the inlet where the pipe connects to the unit [ under the battery tray ] there is a fine mesh filter which possiblly could be blocked.
there IS a 12 pages selespeed thread which might aid you in your quest to fix this problem. One member, pg 11 [ ??? ] talks about the accumulator [ large black sphere unit ] been defective & cauisng over-pressurising of the system which cna also lead to failure.
at the front of the battery are several relays, one of the large black realys [ 50 amp ] controls the pump, might be worth swapping it for another
for software goto AlfaDiag Alfa Romeo Diagnostic Software
for a cable send RAREBEAR a Private Message [ PM ]
if its able to select R & N get some-one to do that as oyu listen to things happening within the engine bay.
it might say its selecting R & N but in fact its doing nothing.