vauxhall vivaro clicking when accelerating?

hi all
i was just wondering if anyone could help me with my van, everything is ok like but when say i going down motorway or any road in any gear i take my foot of accelerator it clicks even when i doing a steady 70 it clicks. its just one of those things thats annoying,
its like there is a flap or something loose ?
it doesnt do it whilst im accelerating just when i take foot of accelerator or doing steady speed
any help would be appreciated