I understood that it was going to be relatively loud. It's still not as loud as a plastic body DSLR with the mirror slap, but it's definitely not the quietest. What I found out is that the camera sounds kind of cheap and hollow with the EF-M 15-45 lens attached. This lens has a plastic flange and body and it's collapsible, which means it's probably going to sound cheap and hollow on any camera body.
When I attach a 70-200 f4L IS, it just sounds better. Can't quantify it, but it sounds quieter, more muffled and definitely more solid. No one ever mentions that you can get a quieter camera by using a heavy and dense lens, even though it makes perfect sense. Try this experiment yourself if you have the chance. Some shops still let customers try different combinations of bodies and lenses at the shop.