I had noticed some grinding when I first started the engine, but it went away too quickly to identify. But after a few drives, I noticed (despite the coolant gauge showing low-peg) that the coolant was boiling, even after short drives. When I checked the radiator, it was always cold, even when the engine was hot. And even when the t'stat cover was hot, there was no flow - not even through the heater core, which showed that the pump wasn't moving any coolant. When I remove it, I found a bore plug (apparently knocked in by the previous owner who had installed the RUBBER plugs) had worked its way through the water jacket to the pump, and become friction-welded to the impeller (which was now stopped, with the pump shaft spinning in it). After installing a new pump, the radiator gets hot all the way to the bottom, showing that there is no substantial blockage inside it. The engine runs perfectly now.
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