Just picked up this lens today and discovered the AF noise is a lot louder than I expected, especially after most reviews mention how silent it is. I know there's a rattlesnake noise that's quite common but this is the focusing motor whirring as it changes focus. Is this normal? My Olympus 45mm is dead silent. I've found two old threads mentioning AF noise but couldn't find any conclusion. One mentioned it should be silent while the other said it's normal - though think they got mixed up with the rattle snack clicking.