Nissan Terrano 4 wheel drive

When i drive in 2 wheel drive i get a whizzing sound coming from n/s front wheel. It then starts banging as though trying to go into 4 wheel drive. If i then select 4 wheel drive and put it back into 2 wheel drive its ok. If i stop or reverse and then set off forward again i have to go through same procedure, selecting 4 wheel drive then 2. I cannot simply drive in normal 2 wheel drive or it starts grabbing and banging again. The problem always developes when either the engine is switched off or i reverse i have to go through procedure every time or it simply makes one hell of a noise from front end. Can anyone help?
By the way its perfect in 4 wheel drive.