M104 3.2 Revs hanging onto revs on throttle lift off when driving - not in park or neutral.

[mlaneley]

Hi, when I lift off approaching a junction or just slowing from speed I would expect the revs to die back to around tick-over but the engine holds on to the revs almost like its engine braking, is this normal? I don't remember it always doing that but I am concerned when I dislodged a vacuum hose off the inlet manifold recently I may not have replaced it correctly, other than that I don't have a clue why it does it.. Anyone have any suggestions please?
Thanks
Malcolm