Are you taking exposures longer than 1 second and have long exposure noise reduction set to ON or Auto?
Long Exposures: Capturing the Passage of Time
Long Exposure Noise Reduction can be set to Enable, Disable, or Auto. In the Auto mode, the camera will determine whether or not it's necessary for exposures of one second or longer. When set to Enable, the camera automatically applies Long Exposure Noise Reduction to all exposures longer than 1 second. (Please note that at shutter speeds faster than 1 second, Long Exposure Noise Reduction is ignored, and has no effect – thus, it can be left "enabled" all the time if you like, and it only is applied in actual long-exposure situations.)
Long Exposure Noise Reduction works quite a bit differently than High ISO Noise Reduction, as there really is no loss of detail. There is one sacrifice however: your camera will be unable to make another exposure until the Long Exposure Noise Reduction has finished. The time that takes is exactly as long as the exposure it was reducing the noise for. So, if you make a 30 second exposure, Long Exposure Noise Reduction will take another 30 seconds of processing time, during which time you will be unable to take another photo with your camera.
What's happening is this: your camera makes its exposure for the image. Long Exposure Noise Reduction then kicks in, and charges the camera's imaging sensor for the same amount of time, making a "dark frame." The camera then compares the noise in the dark frame with the noise generated in the image, and removes any noise that is the same. This method is known as dark frame subtraction.
