Quick Answer:
A sharp whistling or squealing that happens only when you run hot water usually means a small restriction or a vibrating part on the hot side of your plumbing. First, run hot-only fixtures and compare to cold, and with the hot tap at a partial flow feel along the hot supply line for any vibration. If the sound continues after these simple checks, have a plumber locate the exact restriction — they can safely pinpoint and fix the source.
Why This Happens
Hot-only whistling is most often caused by a partial restriction or a loose part that vibrates when hot water moves past it. Possible causes include:
- Corroded or worn washers and seats in valves that vibrate at higher velocities.
- Mineral buildup in aerators, showerheads, or narrow passages on the hot side.
- Partially closed shutoff valves or angle stops creating a high-speed jet.
- Valve packing or stem that is loose or damaged and resonates with hotter water flow.
- Hot-side resonance: metal parts expand with heat and can change how they vibrate.
Step-by-Step What to Do
1. Reproduce and isolate the noise
- Run a fixture using hot water only (open only the hot handle on two-handle taps, or set the single-handle mixer to the hot side).
- Immediately run the cold-only side in the same fixture to confirm the noise is truly limited to the hot supply.
- Test several hot fixtures (bathroom sink, kitchen sink, shower) to see if the noise appears in one location or across the house.
2. Check removable parts (aerators, showerheads)
- Remove the faucet aerator and the showerhead screen on the hot side and run the hot water again. Mineral deposits often cause whistles and may stop when the part is removed.
- If the noise changes or stops, clean or replace the removed part.
3. Feel the hot supply line at partial flow
- Set the hot tap to a reduced flow where the whistle is most audible.
- Gently place your hand on the exposed hot supply line and fittings to locate any vibration. Follow the source toward the fixture and back toward the shutoff valve.
- Note: protect your hand from scalding water or hot copper; test carefully and briefly.
4. Check shutoffs and accessible valves
- Locate the fixture shutoff (angle stop) and operate it slightly — opening or closing a small amount may change the sound and help locate it.
- A partially closed or damaged angle stop can create whistling; if you recently worked on a valve, that’s a clue — see **Screeching after replacing angle stop** for a similar scenario.
5. Consider pressure and system-wide issues
- If the noise appears in multiple fixtures, your pressure reducing valve or water pressure may be involved. High pressure or rapid changes can cause whistling.
- A plumber can measure and adjust system pressure safely.
6. Check nearby devices
- Some appliances or toilet fill valves make noise on the hot line. If a toilet or appliance is involved, compare sounds — a similar problem is discussed with toilets in **Toilet squeals while refilling**.
7. Call a plumber if the sound persists
- If you can’t stop the whistling with the steps above, or if you find vibration inside the wall or at the water heater, contact a licensed plumber to locate and fix the restriction safely.
What Not to Do
- Don’t drain the heater hoping the noise disappears; that won’t address a hot-side restriction and can introduce other risks.
- Don’t force shutoff valves that won’t move — you can break the valve or cause a sudden water surge.
- Don’t remove or tamper with the water heater’s relief valve or internal parts unless you’re qualified; hot-side resonance should be checked by a plumber.
When to Call a Professional
- The noise continues after you’ve isolated fixtures and checked aerators, angle stops, and partial-flow vibration.
- You detect vibration inside walls, ceilings, or the water heater area, or you’re unsure where the noise comes from.
- You’re uncomfortable working on valves or testing while hot water is running; a plumber can pinpoint and repair the restriction without risking burns or damage.
Safety Notes
- Always be careful of scalding. Test water temperature and avoid prolonged contact with hot pipes.
- When feeling pipes, use brief touches and consider a towel or glove on very hot surfaces.
- If you smell gas, have a gas appliance concern, or the heater itself seems to be the source, stop and call a licensed professional immediately.
Common Homeowner Questions
- Is this dangerous? Not usually, but persistent vibration can damage fittings over time; have it checked if it continues.
- Can I fix it myself? Often you can by cleaning aerators or adjusting an accessible angle stop, but a plumber is wise if the source is inside walls or the heater.
- Will lowering the water heater temperature help? It may change how parts vibrate, but it won’t fix a physical restriction; don’t rely on that as the main solution.
For more related articles, see the Whistling or Screeching Noises in Pipes hub.