knocking/something dragging sound on 2005 Chevrolet Trailblazer

My trailblazer is giving a knocking/something dragging sound. It is especially noticeable at low speeds. I got out and looked under the hood while I left the engine running. It looks like the fan, up against the grill in the front of the car is loose/rocking back and forth. Does anybody know what this could be or do I just need to tighten some bolts. Also I have noticed that sometimes while I idle at a red light the car will shake. Any ideas? Thanks for your help in advance.

Asked by for the 2005 Chevrolet Trailblazer
You may have a worn out fan clutch or a fan blade that is out of balance.