you can swab the dust off the mirror. the mirror is for you to see through the view finder and out through your lense. but when you raise the mirror the colorful sqaure behind it is your sensor or chip that recieves the info for your picture. that one you have to be careful of. if you have dust or spots on your mirror you will see them in your view finder. if you have spots or dust on your sensor spots will show up on your pictures. you have an auto clean selection in your menu to do that.
overall when you push the shutter button the mirror raises at the rate you set your shutter speed and then what you were shooting records the image to your sensor. the sound you here is the mirror and if you notice the viewfinder goes black as long as the mirror stays up. hope this helps.
Yes, it's normal. They get dirty....actually "filmy".
Use a lens cleaner on a micro fiber cloth and VERY gently wipe it clean. Do NOT press on it! Just lightly wipe it clean.
minha d5100 caiu com a bolsa e as fotos estão saindo pretas, a camera funciona normal só as fotos que estão completamente pretas. se alguem puder me ajudar eu agradeço.