I understand that this clunking sound is normal and I have read that this sound is the IBIS turning on/off. Elsewhere I read that it is the shutter mechanism opening and closing (I also hear the sound even when I have ES turned on and IBIS turned off). However, here's the issue: very occasionally it will quickly repeat 4 times in row when turning on the camera, and in addition, the viewfinder image flashes on/off concurrently. So is this clunk sound actually from the IBIS or the shutter? I need to know what causes the sound so I can determine whether or not this is a problem. If it's IBIS, could it have something to do with some kind of reset? Otherwise, the camera seems to function normally - at least for now.