I will bet the aerial is laying down on the roof. Open the passenger or side door, stand on the door step reach up and lift the aerial up to a 45 degree angle (the base of the aerials are articulated)you should not get any noise now. When the van was delivered to the dealer the transporter drivers oftern lay the aerials flat to avoid catching them on the upper decks of the transporters. Also if you havent removed the aerial (the mast unscrews) when put through a car wash this can also force the aerial into contact with roof.
Martin. Racer 2, December 2009