Diesel missfiring (pinking)loss of power?

engine starts and ticks over ok, drives for a couple of miles then starts to miss fire, and loss of power, which may clear, or turn off and restart it may clear for a while. Carried out test on fuel filter, its in red zone, (it,s booked in for fuel filter change tomorrow), tried draining water from fuel filter as suggested in Hanes W/Sop manual but after turning retaining nut on botton of filter 1 turn no diesel or water discharged, but could hear a noise as if liquid was flowing. When testing fuel filter conduition I turned the ignition off and could hear a loud whiring noise, when in cab with door shut noise level is much quieter. The van is aTransit 350 115 mwb, june 2005 2402 Duratorque.