'Rushing' noise only with the water pump on?
Or only when draining the sink?
We also had a 'rushing' noise on our first trip...under the bathroom sink with the WP on. Crossthreaded/cracked fitting.
Repaired it with Marine Tex
Seems unlikely that the leak is coming UP from under the floor.
Turn pump on/off quickly and inspect as much of the interior plumbing as possible.
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