A friend has loaned me his AF-S 105mm f/2.8 VR micro lens for a try out to see if I want one myself. When I'm shooting close to maximum magnification I switch the lens to Manual focusing and simultaneously twist the focus ring and engage the DOF preview to determine what I want in/out focus. As I do this I can hear a very light feathery click inside the lens when the DOF preview is engaged and I twist the focus ring - something I've never experienced in a lens before. Is this sound just aperture blades adjusting according to focus point or something else? I'm only trying to determine if this is either a fault in this copy or if it's a trait for this model as I may buy one of these and will possibly inspect lenses being offered for sale.