I have a Nikon 70-300mm VR lens (on a D90) and am not sure if it is performing properly or not. This is difficult to explain (and show)... Occasionally, and I cannot yet identify a pattern, I notice a brief clicking sound and "jagged" movement in the viewfinder as I depress the shutter release to focus. I generally have the VR on. It is very brief and intermittent. The lens functions perfectly otherwise. Is this normal? Any thoughts on what this might be?