From Doc 29-11-08......"When engine driven hydraulic pressure falls below 2600 +/-50 psig or the aircraft flaps are set to greater than approximately 25 degrees, the turbine(ADP) assemblies will automatically start up to provide additional hydraulic pressure."
Now we do have a noise issue during taxi at some major EU airports caused by the ADP. Depending on what the piltos are doing the Hydraulics can get taxed during taxi to Takeoff and the ADPs come on. At idle all the way up to approach power the ADPs are louder than the engines themselves. This happens more often on RR powered airplanes than the GE or the Pratt. Also the RR configuration is different than the GE or the Pratt such that it allows more noise to escape out the boat tail.
From what the hydraulics guys are telling me, flushing a toilet can cause the ADPs to come on.
Oh, there are two configurations on ADP and are operator specified.... The operator can specify 2 per wing or 2 per airplane. Have no info on which configuration is more prominent out there.