Many under-sink reverse osmosis systems send reject water to a drain connection while making filtered water. You may hear that flow while the storage tank refills. A new gurgle, moisture in the cabinet, or water spilling at the faucet air gap calls for a closer look.
Record when the sound begins
Note whether it follows a draw of filtered water, continues when the unit has been idle, or coincides with a slow sink drain. Share a photo of the tubing and visible connection without loosening fittings.
Check the installation against the model instructions
The plumber can assess whether a tube is kinked, whether the receiving drain is restricted, and whether the connection matches the equipment requirements. Tell them about any recent filter, disposal, or trap work.
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