i have a renault traffic van white smoke?

i have a renault traffic van when i start it all seems fine i set off and after about 1 min white smoke come out the when i put my foot down and it dosent mater if it down hill or up smoke comes out but when its parked up and just ticking over and reve it up no smoke comes out. it not to bad on the motorways as im in 6th gear and i hardley have my food down but when i need to pull off in 1st, 2nd or 3rd loads come out can any body point me in the right direction