In the course of a couple of days the Disco went from sounding fine to making some terrible noises. The culprit was the water pump. It somehow failed internally. The impeller blade was still attached to the pulley and was still turning, but that entire assembly was loose in the pump body. With the serpentine belt off, you could easily rock the pulley and fan back and forth. There is not supposed to be any play there.
The noise was from the plastic fan being off-axis and hitting a radiator hose clamp. Thankfully even though the pump was shot, it never stopped pumping water. The engine never overheated and there were no water leaks.
Getting the old pump out and replaced was annoying but reasonable. The coolant had to be partially drained so I topped off the engine oil and coolant after I was done.
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