It sounds like it's on the focusing screen. Turn the camera upside and down and aim the Rocket Blower (large Version) upwards into the Camera. Don't mess with the focusing screen if you're not familiar with it. It's probably around $100 to clean it. If you qualify for CPS, then you can get 2 free clean and checks per year for your $100 gold membership, plus 30% off repairs. You might as well wait for something else to be wrong with the camera before sending it in just for dust, because it will probably happen again.

Whenever you use a Rocket Blower, it's always advisable to just pump it a few times into the air, before blowing it into the camera, this way you will blow out any dust that may be lurking inside the blower.