ACE system bug!

Hello all,
Ive made posts recently regarding ACE/PAS noise on my 2001 TD5. Thanks to searching other threads ive managed to renew ace filter and today fitted a new ace pump. However........i still have a slight droaning noise at low revs and i get a random and regular thud from under the drivers seat while stationary ( I wish it was coming from the CD multichanger but im pretty sure its the ACE!). Im and engineer and work regularly with hydarulic systems and the thud suggests to me - a sudden change in pressure in the system. My question is, has anyone ever experienced a sticky pressure control valve or a defective pressure transducer that hasnt given an amber light? Ive read previous posts about severe fluctuations in pressure but all dead end threads.
Thank you all in advance.

Check this out www.discovery2.co.uk / ACE amber light - Pressure Transducer

Thanks sierrafery
I read this guidance before (excellent stuff). I dont have any amber light though. My only symptoms are the ones described, i.e, strange noises. I would assume the pressure control valve and pressure transducer are supposed to work together to maintain a stable pressure in the ace system. The noises (thuds and vibrations) sounds more like the effects of a relief valve. Im concerned that the PCV has become sticky as i believe this is the only non-serviceable part in the ACE block.
Thanks