Please don't keep us in suspense! Did you have a look?
ChuckFickens is quite right, though I would add that once you've got the case open, I would clean every fan in sight. I know some people suggest using a blower and that can work up to a point, but can just blow the dust into another corner of the case. I use my normal vacuum cleaner and get into the tiny places by taping a small plastic tube to the hose. This gets into the tightest places all around the motherboard and even between the fan blades without placing your hands inside the case too much. One time a wire had come adrift inside the case and was rubbing against one of the fans; a simple job to correct, thank heavens. If you find a particular fan that is the culprit, try to extract it from the case and pop along to Maplins with it, they do a wide range of replacement fans. The connectors are pretty standard. Let us know how you get on.