I have a Nikon DX 50-200 mm lens AF-S that is acting a little strange. Although it works, when you press the button half way to focus, it focuses, then there is a slight clunk and then the picture shifts just a little when you let go of the button without taking the shot.

SOURCE: The auto focus is not working on my Nikon D50

I assume the AF/M switch on the body is set to AF as well? This has not been mentioned but the D50 has two drive systems for AF.

SOURCE: Problem with the Nikkor AF-S DX 55-200 zoom lens.

I just had the same exact problem! However, in my case I was finishing up a photo shoot at work, went to switch lenses, and when I picked up my 55-200mm zoom lens, the tip of it fell off!! I was able to put it back into the end of the lens, but now I am having the same autofocus problem and sound effect. I think it is because it is off track inside the lens. But I have only had mine for 5 months! I'm preparing to send it to Nikon for warrenty repair.

SOURCE: Nikon D80 Autofocus problem

Try re-setting the camera? On the D40 it is 'Menu' scroll down one to Custom Setting Menu (a pencil) and click reset 'ok' yes 'ok' this worked for me.

SOURCE: Problems with Auto Focus on Nikon D100

You can adjust the number of focus areas so that it will not try to compensate for many objects in the pic at 1 time

SOURCE: Nikon D40 will not focus when shutter pressed 1/2 down

has the lens stuck up? put it temporarily on 'M" and try to turn the focus ring. this should be freely turning;

otherwise you may want to check the camera body to lens electrical contacts (silver).. a small dab of alcohol to a q-tip.. make sure its not that wet..then try to clean the contacts.