probably your viscous fan
To test the viscous coupling, you need a cold engine to start with. Get the engine running, and then gently try to stop the fan blades turning with a loosely rolled up newspaper. When the engine is cold you should be able to easily stop the fan.
Now repeat this test with an engine that is significantly overheating. i.e. The temperature gauge needle around the three quarters mark. When the newspaper meets the fan, the newspaper should get shredded. If the fan stops with the same resistance as when cold, then your viscous coupling is broken, and needs replacing.
hope this help's taking from the zone wiki