Ouch, kinda sounds like it could be cavitation, it happened to my diesel
truck. It was running fine but loosing coolant and I couldn't find a leak.
Then it hydrolocked, coolant was leaking through the cylinder walls.

A few links to check out, must are

It will be interesting to see if that could be the cause, but it wouldn't seem
likely.

My cooling system has been routinely serviced, including the very recent
replacement of a heat exchanger. The engine was running *cooler* with the new
heat exchanger, so It would be hard to suspect heat related damage. The engine
has not been using coolant.