Shut Dat Shutter
Set the camera so that it will fire the shutter with the lens off. Put it to manual, and the shutter speed to 1/60 of a second. Take the lens off, and look into the open mount as you press the shutter button.
What the heck was that!? It was the shutter snapping into place, opening for 1/60 of a second, then snapping shut again, then opening up. If you had put a cigar end in there you could have cut it off...No, no, no - that was just a joke. Don't do that. Smoking cigars is bad for you and even worse for shutters.
The point of this exercise is to show you where the shutter is. It used to be tucked away decently a the back of the camera body just in front of the film plane* or hiding coyly behind a swinging mirror - now it is up the front ready to be broken.
Reduce the chances of this - now you've done the experiment, don't repeat it. If you are going to clean your own sensor ( Questionable decision ) don't leave the camera on ( Bad decision) and then press the shutter button ( Terrible decision ). Repairs are profitable but only for the repair shop - not for you.
If you are concerned about dust or smudges on your sensor past what the little cleaning program of your camera can cope with, bring it in to our repairs department. We can do it well, quickly, and without busting the shutter.
* Was a PRU Spitfire considered a film plane...?