hi shaun
i had a fault with this system this year which turned out to be a blocked exhaust valve on compressor pump in tyre well,the valve in question is the brass ball composition,remove valve & drill a cross hole in it or just test system with it removed,it is only there to quiten down,there was a special bulletin on this fault from land rover in which they sell you a new exhaust valve as i described & a new drier filter & another part which i cant remember right now
hth colin