Most modern (late 3rd and 4th generation) pressuriesed water-water reactors have the ability to operate under natural circulation when on low power.
In liquid metal reactors you could use magnetic pumps which do not have any moving parts, thus the only noise you make is the flow noise (which is more or less constant in this case and can be actively cancelled out).
Another means of reducing the noise of a nuclear submarine is to detach the turbines from the propulsor, for example by using electric transmission. Partially this is done on the pr.885 for example - when on low speed it would travel under the electric motor.
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