Why Does My Pool Pump Make a Gurgling Sound After Backwashing?

You might not know that a gurgling pool pump after backwashing can tell you quite a bit about what is happening inside your filtration system. A little bubbling or sloshing immediately after restarting the pump may simply mean trapped air is moving through the plumbing. But when the sound continues, gets louder, or comes with weak water flow, it can signal a problem that deserves attention. The key is figuring out whether your equipment is clearing temporary air pockets or struggling to maintain a steady supply of water.

Backwashing changes the direction of water flow through certain pool filters, flushes dirty water out through the waste line, and requires the pump to stop and restart. Those changes can expose existing suction problems or create temporary conditions that make the pump sound different. Here is how to identify the likely cause without unnecessarily taking apart your equipment.

What Happens to Your Pool Equipment During Backwashing?

In normal filtration, the pump pulls water from the pool through the skimmer and other suction lines, pushes it through the filter, and sends it back through the return jets. During backwashing, water moves through the filter in a different direction to remove accumulated debris and carries that dirty water out through a waste line.

When the system switches back to normal filtration, water must once again establish its usual circulation pattern. Air trapped in the filter or associated plumbing may move through the system during startup.

However, backwashing does not automatically explain persistent air inside the pump basket. If bubbles continue collecting beneath the clear pump lid, consider whether the pump is drawing air from the suction side rather than simply clearing air left in the filter.

1. Your Pool Water Level Dropped During Backwashing

This is one of the easiest causes to overlook. Backwashing sends pool water out of the system instead of returning it to the swimming pool. A long backwash cycle, followed by rinsing, can lower the water level enough to affect skimmer operation.

When the water level falls too low, the skimmer may draw air along with water. You might notice a small whirlpool forming inside the skimmer opening, accompanied by gurgling at the pump.

Check whether the pool water is at the level recommended for your skimmer, commonly around the midpoint of its opening. If the skimmer is drawing air, restore the appropriate water level before restarting the system.

A useful clue: If the pump sounded normal before backwashing but began gurgling after the pool level dropped, inadequate water supply at the skimmer may be more likely than an internal pump failure.

2. Air Is Trapped in the Filter or Plumbing

Some air accumulation during shutdown and startup can occur. When circulation resumes, the moving water may carry air pockets through the system, producing temporary bubbling or gurgling.

Look at the clear pump strainer lid, if your pump has one. A small amount of air that clears as the pump primes is different from a large pocket that continually grows while the motor runs.

Many pressurized pool filters have a manual air relief valve designed to release trapped air. Follow your specific filter manufacturer's startup instructions, stand clear of the filter, and use the air relief only as directed. Never loosen the filter clamp, lid, or other pressure-containing parts while the system is pressurized.

If the sound stops after the system clears its air and the return jets produce steady flow, no additional repair may be necessary.

3. The Pump Lid O-Ring Is Allowing Air Into the System

A pump can draw air through a small opening on its suction side without producing an obvious water leak. This happens because the plumbing upstream of the running pump operates under suction.

The clear strainer lid is a common place to investigate. Its O-ring may be dirty, twisted, dry, worn, or improperly seated. A lid that was recently removed to empty the pump basket may also have been reinstalled incorrectly.

With the equipment switched off and the system safely depressurized according to the manufacturer's instructions, inspect the accessible lid seal. Remove debris from the sealing surfaces and check for cracks or flattening. Replace damaged seals with the correct part, and use only a manufacturer-approved lubricant if recommended.

A loose drain plug, deteriorated plug seal, or suction-side fitting can produce similar symptoms. If the pump continues pulling air despite a properly sealed lid, professional leak testing of the suction plumbing may be needed.

4. The Pump Has Not Fully Reprimed

Priming means filling the pump and suction pathway with enough water to establish normal circulation. A pump that has lost prime may produce a hollow, uneven gurgling sound as it struggles to move water consistently.

You may see the water level rise and fall beneath the pump lid instead of remaining relatively stable. The return jets may also produce weak or intermittent flow.

Long suction lines, equipment installed above pool water level, and air entering through an imperfect seal can make priming more difficult. Variable-speed pumps may also behave differently at lower startup speeds, depending on their programmed priming settings.

Use the pump manufacturer's priming procedure rather than repeatedly running the motor and hoping the sound disappears. Never allow a pump to operate dry. If it cannot establish prime within the manufacturer's specified time, shut it down and investigate the cause.

5. A Dirty Skimmer Basket or Suction Restriction Is Limiting Flow

Backwashing cleans the filter media, but it does not clean every part of the circulation system. A skimmer basket packed with leaves, an obstructed pump basket, or a restricted suction line can still reduce the amount of water reaching the pump.

Reduced incoming flow may create turbulence, unstable priming, and unusual noise. In more serious situations, inadequate water supply can contribute to cavitation, which often sounds more like rattling or gravel than simple bubbling.

Check the accessible skimmer basket for debris. Inspect the pump basket only after shutting off the equipment, isolating it as required, and releasing pressure using the manufacturer's procedure. Never reach into an active suction opening or operate the pump with suction safety components missing.

If both baskets are clear but the problem persists, avoid experimenting with suction valves or disconnecting plumbing while the pump runs. An obstruction deeper in the system may require professional diagnosis.

6. The Multiport Valve Was Not Returned to the Correct Position

For sand and certain diatomaceous earth filters, the multiport valve determines how water travels through the system. After backwashing, the valve must be returned to its normal FILTER setting using the correct procedure.

A typical sand-filter sequence is BACKWASH, RINSE, and then FILTER, with the pump turned off before every valve-position change. Rinsing helps settle the filter media and sends remaining dirty water to waste. Some slide-valve systems use different procedures and do not have a rinse setting.

A valve that is not fully seated, has a damaged internal gasket, or leaks between internal passages may interfere with circulation. One overlooked warning sign is water continuing to flow from the backwash waste line after the valve has been returned to FILTER.

That symptom does not automatically mean the pump is damaged. It may indicate a worn spider gasket or another internal multiport valve problem. Have the valve inspected if the waste line continues discharging during normal filtration.

How to Troubleshoot Gurgling After Backwashing

Rather than changing several things at once, use a simple sequence to narrow down the problem.

Do not treat every pressure reading as interchangeable. A lower pressure after successful backwashing is often expected because the filter offers less resistance to flow. Low pressure combined with weak circulation and air in the pump, however, can suggest an inadequate water supply or suction restriction.

Does Gurgling Mean the Pool Pump Is Damaged?

Not necessarily. A short period of bubbling after startup does not automatically mean the motor, impeller, or pump housing has failed.

The sound becomes more concerning when it is accompanied by poor circulation, repeated loss of prime, unusual vibration, grinding, overheating, or a pump that will not move water reliably.

It is also important to distinguish between water-and-air gurgling and mechanical noise. A pump that produces a metallic screech, harsh grinding, or repeated rattling may have a problem unrelated to trapped air, such as bearing wear or cavitation.

Shut the equipment off if it runs dry, makes severe mechanical noise, overheats, or cannot maintain circulation. Continuing to operate a malfunctioning pump can turn a relatively minor problem into an expensive repair.

What If the Pool Also Seems to Be Losing Water?

Backwashing deliberately removes water from the pool, so a lower water level immediately afterward does not by itself indicate a leak. Mark the water level only after refilling to its normal operating range, and distinguish intentional backwash discharge from water loss during ordinary filtration.

If the pool keeps dropping after backwashing is finished, check for ongoing discharge from the waste line and other visible leaks. Water loss through a faulty multiport valve can occur without an obvious puddle near the equipment.

For unexplained water-level changes, the Mini Bucket Test can provide a simple first-step comparison between normal evaporation and possible leak-related water loss. It cannot identify a leak's location or replace professional diagnosis, but it may help determine whether further investigation makes sense.

When Should You Call a Pool Professional?

Professional help is appropriate when the pump repeatedly loses prime, suction-side air persists after basic checks, or the filter and valve show signs of malfunction.

Arrange an inspection if you notice these patterns:

• Gurgling continues during normal filtration rather than clearing after startup.
• The pump basket never fills properly or repeatedly develops large air pockets.
• Return flow becomes noticeably weaker after backwashing.
• The pump produces grinding, rattling, or other severe mechanical noises.
• Water continues escaping through the waste line while the valve is in FILTER.
• Filter pressure behaves abnormally or the system cannot be safely primed.

A technician can evaluate suction-side air leaks, valve seals, pump performance, and plumbing restrictions without relying on guesswork. Pressurized filter repairs and electrical troubleshooting should be handled by qualified professionals.

How to Prevent the Problem Next Time

Before backwashing, check the water level, remove debris from accessible skimmer baskets, and make sure the backwash discharge route is unobstructed. Follow the filter manufacturer's backwash duration and rinse instructions rather than assuming every system uses the same cycle.

Always shut the pump off before moving a multiport or slide valve. After returning to normal filtration, release trapped air using the approved procedure, verify steady return flow, and record the clean-filter pressure for future comparison.

Most importantly, pay attention to how the equipment normally sounds. A familiar, steady pump makes it much easier to recognize when air, restricted flow, or mechanical trouble changes its behavior.