I've been working on a water cooled build. As my room mate and I were bleeding the system, the pump started to rattle. We though it had caught some air. Way we ended up doing was loosening an extra plug on the discharge. A few drops of liquid came out, no air. I removed the pump from the loop and just made it pump water from the sink, no rattle and a lot of flow, good pressure. It looks cavitation to me. Any other thoughts? And how do we deal with it?