Short answer
Start by confirming whether the sound is tied to actual water flow. Turn off all fixtures and appliances, then check your water meter’s low-flow indicator. If it’s moving, you likely have a supply-side leak. If the meter is still, the noise may be drainage, heating, rain in the vent stack, or normal fixture operation. Isolate sources one by one (toilets, ice maker, irrigation, humidifier), listen with a stethoscope, use a moisture meter to scan walls, and only open the wall after you’ve narrowed the area.
What that running-water sound might be
Common causes include:
- Supply leak (pinhole in copper, PEX fitting, slab leak)
- A toilet slowly refilling due to a leaky flapper or fill valve
- Irrigation or humidifier supply bleeding water
- PRV/expansion tank issues causing constant flow or hissing
- Drain/vent noise when appliances dump water (dishwasher, washer)
- Hydronic/radiant heating circulation
- Rainwater in vent stacks or downspout pipes in walls
- Condensate drain from AC/furnace
Tools and materials
- Flashlight and inspection mirror
- Mechanic’s stethoscope (or a long screwdriver as a listening probe)
- Moisture meter (pin or pinless)
- Infrared thermometer or compact thermal camera
- Painter’s tape and notepad/phone for mapping
- Food coloring or toilet dye tablets
- Adjustable wrench and screwdriver
- Non-contact voltage tester and stud finder with AC detection
- Towels, bucket, utility knife; PPE: gloves, eye protection, N95 if opening walls
Step-by-step diagnosis
1) Establish whether water is flowing
- Turn off all faucets, showers, outdoor spigots; pause dishwasher/washing machine; let ice maker finish a cycle.
- Check the water meter:
- Find the low-flow “leak” indicator. If it spins with everything off, you have a supply-side flow.
- Record the meter reading; wait 15 minutes with no water use. Any increase indicates flow.
- On a well system, listen for pump cycling when no water is in use. Intermittent cycling suggests a leak.
Normal residential pressure: 40–60 psi
High pressure (may cause noise/issues): >80 psi
2) Isolate fixtures and appliances
- Toilets: Put dye tablets or a few drops of food coloring in the tank, wait 10 minutes. Color in the bowl = leaking flapper. Also listen at the fill valve for a faint hiss.
- Shut off valves to: refrigerator/ice maker, humidifier, water softener/RO system, irrigation/backflow. Re-check the meter after each shutoff.
- If noise stops when a specific valve is closed, you’ve found the branch causing flow.
3) Differentiate supply vs. drain/vent
- Close the main water shutoff. If the noise stops, it’s supply-side. If it continues, it’s likely drain/vent, rain, or hydronic circulation.
- If the sound coincides with dishwasher/washer dumps or tub draining, it’s probably normal drain noise or a loose drain pipe not isolated with soundproofing.
4) Locate the area
- Use a mechanic’s stethoscope to listen at baseboards, around plumbing chases, and near fixtures. The loudest point is typically within 1–2 feet of the issue.
- Scan for moisture: run a moisture meter along baseboards and 12–24 inches up the wall. Mark high readings with painter’s tape.
- Use an IR thermometer/thermal camera to check for cool or warm anomalies (cool for cold-water leaks; warm spots for hot-water or slab leaks). On concrete slabs, a warm streak can indicate a hot-water slab leak.
5) Consider non-plumbing sounds
- Hydronic/radiant heat: If noise aligns with heating cycles, it may be circulation through baseboards or radiant loops. Air in lines can cause rushing sounds.
- Rain events: During rain, vent stacks can transmit water sounds. Normal, but can be loud in poorly isolated walls.
- HVAC condensate: Running water sounds near the furnace/air handler can be condensate draining. Inspect for clogs or cracked lines.
6) Open the wall only after narrowing
- If moisture and sound are localized, cut a small inspection opening (4x4 to 6x6 inches) between studs at the loudest/moist area. Use a stud finder and non-contact voltage tester first. Be ready with towels and a bucket.
- If an active leak is visible, shut off the main immediately and capture water.
Safety considerations
- If you see or suspect water near electrical outlets, switches, or fixtures, switch off the circuit at the breaker before opening the wall.
- Use gloves, eye protection, and a mask when cutting drywall (mold risk).
- If temperatures are below freezing and you hear rushing after thawing, be prepared for a burst line; keep the main valve accessible.
Quick fixes and what they cost
- Toilet flapper/fill valve: $10–$35; 15–45 minutes per toilet.
- PRV adjustment/replacement: $0 to adjust; $75–$150 for a new PRV plus labor.
- Expansion tank (if water heater on closed system): $50–$120 plus $100–$200 labor.
- Moisture meter: $30–$80. Mechanic’s stethoscope: $10–$25. IR thermometer: $25–$50.
- Professional leak detection: $300–$600. Slab leak repair: $500–$3,000+ depending on access.
Tips for best results
- Work systematically and take notes. Mark test points with painter’s tape.
- Test at quiet times (night/early morning) to better hear changes.
- Don’t overlook toilets—small hisses there are the most common hidden water use.
- Check irrigation. A slow leak between the backflow and the first zone valve won’t show indoors but will run constantly.
Common mistakes
- Cutting large sections of wall before confirming the zone.
- Assuming all water sounds are leaks—many are drains or HVAC condensate.
- Ignoring high water pressure; it can cause constant valve noise and accelerate leaks. Consider a pressure gauge on an outdoor spigot.
- Not rechecking after each isolation step; change only one variable at a time.
When to call a pro
- The meter shows continuous flow and you can’t isolate it within 1–2 hours.
- You detect damp walls, musty odor, or visible mold.
- Suspected slab leak (warm floors, meter spinning, no visible moisture).
- Noise or moisture near electrical components.
- Multifamily/condo with shared systems or limited access—avoid liability.
With a methodical approach and a few affordable tools, you can usually separate harmless sounds from true leaks and pinpoint where to open the wall—or gather the right info for a plumber to resolve it quickly.