Anyone know about ACE on discoverey 2004-am getting a loud hissing sound everytime I halt at a junction and when I finally park-all suspension seems to working ok apart form the hising soudns like a lorry air brakes or cold liquid hitting hot pipes No signs of leaks

I have a 2004 LRD TD5 Metroplis-when I stop at junctions and finally park there is a lot of hissing like a lorry air brakes Dealer says it is the ACE system-Is this normal to hiss every time