Mine was doing exactly the same thing. The pulley was all over the place, but no leaks at all. Took the water pump off and the bearing was completely toast and the impeller was hitting the casting, making the knock. I replaced the pump and all is good, but now the CEL came on? I hope it's just low coolant? I need to check the code.

The crazy thing with mine, is that the car on has 35 k on it....