Either you got a nasty case of trapped air in the system, which will require a full system bleed...
...OR...
The bearing in the water pump is on it's way out, I think there is a way of checking pump play, the shaft has a cover, which allows you to turn the pump using a screwdriver, you can check to see if it is grumbly. If the pump is on its way out, it is replaceable DIY as long as there is a shut-off valve above and below the pump.
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