My Sigma 30mm/2.8 arrived today. It matches what everyone else has been saying - nice optics, slow startup, quick focus, sturdy but plastic-y build, great price, etc. But it wobbles unless it's mounted on the camera and powered on. Aim above level and it feels like something inside slides backward. Aim below level and something wobbles forward. It's not a minor looseness. Something with a bit of heft to it is moving around in there. In my mind I'm picturing a floating optical group of lenses wiggling about. This doesn't happen when the camera is powered on. It seems to operate just fine during use. So weird!! Has anyone else noticed this? Do I have a bad copy?