My MBP is making funny noises when I turn it on. It's been happening a month or two now. I've assumed it's dust in a fan which gets dislodged after time. As it becomes more regular, it seems vaguely connected with temperature. After the MBP has been on a while, the noise goes. This evening, I noticed that it's directly related to the brightness of the screen; if the screen is off or very low, the sound is less bad; towards middle-brightness, the noise is worse; and at full brightness it's completely absent. Does anyone know what's causing this? Should I worry about it, or is there anything I can do?