My iMac (27-inch, Late 2013), is SHEDDING! Dust comes out of the vent openings on the backside. I did vacuum the vent holes without opening them, when I noticed dust particles dropping onto my desk. Now about 10 days later, the dust is back. My Imac is dropping its droppings onto my white desk again. Is this normal? I should mention that this is a recent situation. There was no visible dust until about two months ago. The computer is 3 years old.