?tappet noise on 2000 996 cab - on run off in 3rd or 4th

There was a ticking sounding like a tappet noise and when Porsche looked at it they said that it was the vraiable valeve timing units sticking and put new ones in. Now instead of a single "tick" it sounds like all of the tappetys are clicking - only happens on coastui downhill in 4th mainly but cant hear it when accelerating (maybe exhaust a bit loud). Porsche have taken the manifolds off,head off and camshafts out to replace timing units so should have seen anything else(??) - any ideas what this may be. Oil pressure is fine and always has been -starts on 5 when cold and then on tick over goes down to just under 3 on dash gauge(I know this is not the most reliable) btut has not changed prior to the noise to now Thanks Simon