Dyson D08 keeps losing suction ?

Our Dyson D08 has worked great since we got it maybe 4 years ago but recently it started "popping" the internal valve that should only activate in the event of the inlet becoming blocked. You here a "pop" noise and machine still runs but at a greatly reduced rate of suction. This system worked great as it should when ever you tried to suck up something you shouldn't but now seems like valve has softened off and pops all the time we use it and preventing us from cleaning properly. We have checked / cleaned all the obvious things .. any idea's ??