This is where the focusing screen is (ignore the magnified section)...
Never use cotton buds to clean the inside of the camera. If you want to clean it you ought to buy some proper cleaning products - they don't have to be expensive and you won't spread dust, oil or solvents around the inside.
Get yourself a proper
blower (£8), some
PEC pads (£10) and some
ROR fluid (£4). Use the blower to clean out the inside of the camera (around the mirrorbox), drop a small amount of ROR fluid onto a PEC pad and carefully wipe the focusing screen.