Citroen Berlingo clicking noise from outside when vehicle is in motion

Any Suggestions :- I have a clicking noise coming from some where outside my berlingo when it's on the move. I have checked all the tyres they are all ok, as i speed up or slow down the clicking also speeds up and slows down. There is no clicking when the vehicle is stationary. It make no difference if the clutch is depressed or not. I had a 50 mile motorway journey and it seemed to start when i came off onto the slip road. Thanks Glen