I recently got the Olympus 75 f1.8 for my e-m10. I did some initial testing and everything seems to be pretty good producing a nice and sharp image. Anyways I didn't notice this until I was in a quiet room I started to hear some kind of grinding/Mechanical noise. For example, when I use Aperture mode I would hear the grinding/mechanical noise every time the shutter speed jumps from lets say 160 to 4000 and even small jumps I would sometimes hear it. I was wondering if this is normal for this lens or Is it a bad copy?