I had the issue. I had my wife turn the water on while I watched under the sink. It was the connection of the plastic tube to the outlet attached on the outside. Specifically, the female connector is too long and it runs out of threads on the male connector of the outlet. Therefore it doesn't seat the washer and no matter what, it was loose. I went to the hardware store and bought a flexible hose of the same length with the proper female connectors (one was metal one was plastic) with built in washers. snugged right up $4.79 solution.
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