Toilet Makes a Foghorn Noise When Refilling? Diagnose the Fill Valve

Common Symptoms

• A low horn, moan, or vibration starts after flushing and stops when the tank reaches its fill level
• The toilet refills slowly, pulses, or makes a changing pitch as the water level rises
• The sound becomes worse after water has been shut off for plumbing work or after a supply interruption
• Water continues to trickle into the overflow tube or the fill valve sputters near the top of the refill cycle

Most Likely Causes

1. 1

   Worn or Mineral-Clogged Fill-Valve Seal

   Many modern fill valves use a flexible cap seal that vibrates when grit catches under it or when the rubber hardens with age. That vibration travels through the plastic body and tank water as the familiar foghorn sound.

2. 2

   Partly Closed or Restricted Supply Stop

   A stop valve that is not fully open can create a high-velocity restriction. Old multi-turn stops can also shed debris internally, so the toilet may receive uneven flow even when the handle appears open.

3. 3

   Float or Refill Adjustment Problem

   If the float is set too high, the valve can stay open longer than intended or send water into the overflow tube. The extended run time makes an already noisy valve more noticeable and wastes water.

Quick DIY Checks

1. 1Remove the tank lid and flush once while listening from a safe, dry floor. Confirm that the sound begins only when water enters the tank. If water rises into the overflow tube, turn the shutoff valve clockwise to stop the refill and address the high water level before testing anything else.
2. 2Check the supply stop behind the toilet without forcing it. If it is a modern quarter-turn valve, verify its handle is parallel to the pipe. For an older multi-turn valve, do not force a seized handle; a corroded valve can begin leaking at the stem and should be handled by a plumber.
3. 3Turn off the toilet supply, flush to empty most of the tank, and place a towel below the valve. Follow the fill-valve maker's instructions to remove and rinse the cap seal. Use clean water only; abrasive cleaners can damage the sealing surface and make the valve louder.

Repair vs Replace

Recommended Tools & Parts

• Universal adjustable toilet fill valve

  Choose a valve compatible with your tank height and follow its instructions for float adjustment and refill-tube placement.

  $15–$35

  Buy on Amazon →

• Fill-valve cap seal

  Use the seal made for the exact valve model when a removable cap is available; similar-looking seals may not seat correctly.

  $3–$8

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

Related Repair Guides

This section combines editorial `relatedSlugs` from the article source with metadata-based matches so each guide links deeper into the repair library.