Water pump rattling when I start car. After the car warms up, the noise stops.
It sounds like the impeller shaft inside the water pump has been bent or broken. If this were the case the noise would exist when the engine was cold, then when warmed up and coolant starts to pass through the water pump, the coolant may provide just enough lubrication to stop the noise. I would recommend getting your water pump replaced by a professional to prevent any metal shavings or other debris from contaminating the rest of your cooling system.