I took a pretty close look at the water pump today and it sounds like the sound is definitely coming from it. The fan clutch and shaft feel good and tight, there is small wiggle that's barely detectible so I think this might be normal? It only wiggles when I try to tilt the fan back or forth and it's a very small amount. The rattle only appears when the engine is warm and seems to get worse the more coolant is being pumped and disappears above 2000 rpm. I'm pretty baffled at this point. It seems like if it were a bearing it would not be temperature dependent but I could be wrong. Also, I let the engine warm up to the point where the fan kicked in and still rattles so I don't think it is the clutch.