When I turn off my digital Camera Nikon D-90 its body makes a little noise is it normal?

Unless you've dropped it, the noise is most probably the image sensor cleaning itself. If you look into the front of the camera when there is no lense attached, you'll see a small rectangle set at abot 40 degrees, this is the sensor. when you turn of the D90, it vibrates the sensor to remove any dust that may be on it. Hope this helps :-)