The shutter sound is a selectable sound just like any other, meaning it's not mechanical, it's just the default sound for that app. With that in mind, do any other apps have this sound as an alert - like text or email? It's possible your texts emails or some other alert has this or a similar sound and you receive them that's the noise it's making. Also, is the camera app closed? Sometimes the aperture will actually make a noise as it tries to focus.
You should be able to swipe down from the top and see recent notifications.
I appreciate the functions that new phones have but they make them waaaay too complicated. I am probably only aware of about 1/10th of the functions my phone has. And when issues arise like this, you have to be an engineer to figure out how to fix it.
