If your under-sink reverse osmosis system makes a sharp bang in the pipes whenever a faucet shuts off, the sound has a name: water hammer. It happens when moving water stops suddenly and the pressure wave has nowhere to go. Many St. Louis homes already have a knock or two from a dishwasher valve or an ice maker fill line, so a new RO install can make an old habit easier to hear. Here is how to sort out what you are hearing before you touch anything.
What Water Hammer Sounds Like Under the Sink
Water hammer usually shows up as a short, hard thud, often felt through the cabinet floor or the back wall. A softer hum, buzz, or rattle points to a different problem, such as a loose line or a vibrating valve. Pay attention to when the sound happens. Note whether it comes right after you close the RO faucet, when the storage tank fills, when the kitchen faucet shuts off, or when another appliance stops running.
Checks to Run Before Calling for Help
- Look at the cold feed line and the drain line. Make sure neither one rests against a pipe, a stud, or the back of the cabinet. A line pressed against framing carries every knock straight into the house.
- Check the clamps and fittings near the storage tank and the automatic shutoff valve. A fitting that feels snug by hand can still back off over time, and a loose joint can rattle hard under pressure.
- Identify which fixture triggers the sound. If the knock only happens when you close the RO faucet, the problem is most likely in the line running to that faucet or at the faucet itself.
- Run the dishwasher and the ice maker one at a time while you listen. If the knock matches one of them, the shock is coming from that fill valve rather than from the RO system.
- If you have a pressure gauge, check the house water pressure at an outdoor or utility spigot. Pressure on the high side of normal makes water hammer worse, and a gauge is the only reliable way to know where your home stands.
- Look for a water hammer arrestor on the main supply or at the fixture. An arrestor is a small chamber that absorbs the shock. Some St. Louis homes have one already, and some do not.
Fixes Homeowners Can Often Handle
If the lines are loose, secure them with cable ties or pipe clamps that have a rubber or foam liner. Keep the lines away from framing so they do not transfer vibration into the wall. Snug any fitting that has backed off, but do not overtighten plastic fittings, since that can crack the housing. Foam padding between the line and the cabinet back is a simple way to quiet a line that rubs during flow. Clear the space under the sink of loose bottles or cleaning supplies that bounce when the cabinet shakes.
When to Stop and Call a Plumber
Stop and call a licensed plumber if you see drips at any fitting, if the sound is loud enough to shake the cabinet every time a fixture closes, or if the pipes move visibly inside the wall. Repeated hard hammer can loosen joints and wear out valves, and those repairs are easier when caught early. A plumber can check whether the home needs an arrestor on the main supply line. Describe exactly which fixture causes the sound and when, since that detail speeds up the diagnosis.
Keeping the System Quiet Over Time
Recheck the clamps and fittings during your first few months with the system, and again each time you change filters. Write any new sound in your maintenance notes along with the date and the fixture that triggered it. A system that suddenly starts knocking after a quiet period often points to a fitting that loosened over time or a valve that is beginning to fail. Catching that change early is usually simpler than tracking it down after a leak appears.