Cavitation is the sound of a pump trying to move more water than the suction side can feed it. Vapor pockets form at the impeller and collapse — it sounds like the housing is full of gravel. The fix is almost always on the suction side: raise the water level, clean the baskets, open every suction valve fully, and seal any air leak. The motor is rarely the problem.
What Cavitation Actually Is (and Why It Wrecks Impellers)
Here’s the short version, because it’s the part most pool sites skip: a pool pump doesn’t suck water, it removes pressure so atmospheric pressure pushes water in. When the suction side can’t keep up — low water, a clogged basket, an air leak — the pressure at the impeller drops far enough that the water flashes to vapor. Those vapor bubbles travel into the high-pressure side and collapse, hammering the impeller vanes thousands of times a second. That collapse is the gravel sound, and it’s also what slowly chews pits into the impeller. About 90% of the time I trace it to a suction-side restriction, not the motor. Fix the suction side and the noise stops the moment water flow is restored.
I spent 19 years on F-16 fuel and hydraulic systems in the Air Force, and cavitation was the enemy there too — a hydraulic pump starved for fluid does the exact same thing and fails the exact same way. So when I hear that sound at a pool pad, I don’t reach for a motor. I walk the suction side first. Here’s the assumption that costs people money: almost everyone hears that gravel rattle and decides the bearings are shot or there’s a rock in the basket. Nine times out of ten it’s neither. The bearings are fine and the basket might be clean — the pump is simply starved for water. Early in my food-service repair years I’d have called a rattle like that ‘bad bearings’ and quoted a new motor — it took me a few embarrassing teardowns to learn the sound is almost always starvation, not worn steel.
Chlorine King Pool Service walks through telling cavitation apart from a clogged impeller — same diagnosis covered here.
The Causes, Most Common First
Work these in order. The first three are free and fix the large majority of cavitation I see. Don’t jump to plumbing changes or a new impeller until you’ve ruled these out — I’ve watched people re-pipe a suction line that only needed two inches of water added to the pool.
1. Low Water Level Starving the Skimmer — About 40% of Cases
This is the big one, and it’s free. When the water drops below roughly the midpoint of the skimmer mouth, the skimmer starts pulling air and water in gulps instead of a steady column. The pump gets a slug of water, then a slug of air, and cavitates on every air slug. Walk over and look at your skimmer right now. If the water line is at or below the bottom of the opening, add water with the garden hose until it sits mid-mouth, then listen. The gravel sound usually clears within a minute. I’ve fixed more ‘broken’ pumps with a garden hose than with any tool in my bag. I keep a float valve on my own pool for this exact reason — once July hits I was losing an inch a day to evaporation and got tired of topping off by hand.
2. Clogged Strainer or Skimmer Basket — About 25% of Cases
A basket packed with leaves and hair chokes the flow before it reaches the impeller. The pump pulls hard against the restriction, pressure drops at the impeller, and it cavitates. Pop the pump lid, pull the strainer basket, and clean it. Then check the skimmer basket too — people clean one and forget the other. While the lid is off, look at the lid o-ring; if it’s flat, cracked, or stiff, that’s your next suspect. If you want the full rundown on suction-side noise, the pool pump noise guide breaks down every sound by cause.
3. Air Leak on the Suction Side — About 15% of Cases
An air leak lets air into the line ahead of the pump, and the pump cavitates on the air the same way it does on a starved skimmer. The tell here is bubbles: you’ll see air collecting in the clear pump lid while it runs, and bubbles coming out of the return jets in the pool. Run the shaving cream test — smear a little on each suction-side fitting, the pump lid, and the drain plugs while the pump runs, and watch for the cream to get sucked into the leak. The air bubbles guide covers every spot air sneaks in.
4. Pump Too Big for the Plumbing — About 12% of Cases
This one’s counter-intuitive and it’s the field insight most homeowners never hear: a pump can be too powerful for the pipe feeding it. A 2 HP pump bolted to 1.5-inch suction plumbing will try to pull more water than that pipe can physically deliver, so it cavitates even with clean baskets and a full pool. A inch-and-a-half line tops out around 45–50 GPM before friction chokes it. If your pump was recently upsized and it’s cavitated ever since, that’s the cause. Dropping a variable-speed pump down to a lower RPM fixes it instantly — less demand, no starvation. On a single-speed pump, the real fix is bigger suction pipe or a smaller impeller.
5. Partially Closed Suction Valve — About 8% of Cases
A suction-side valve that’s only half open does the same thing as undersized pipe: it restricts flow and starves the impeller. Walk the suction plumbing between the skimmer/main drain and the pump and confirm every valve is fully open. A valve cracked halfway can cut flow by 70%, and that’s plenty to cavitate. This takes thirty seconds to check and it’s embarrassing how often it’s the answer (don’t ask me how I know).
How to Confirm It’s Really Cavitation
Before you spend a dime, confirm the diagnosis with this trick I lean on every time. With the pump running and making the noise, briefly cup your hand over part of the skimmer opening to throttle the suction. If the pitch of the sound changes instantly — gets louder or quieter the moment you restrict the skimmer — it’s a suction/flow problem, which is cavitation. Bearing noise doesn’t react to what you do at the skimmer; it’s tied to the spinning shaft and stays constant. That one test separates a $0 fix from a misdiagnosed motor more reliably than anything else I know. I’ve walked people through this same hand-over-the-skimmer test on the phone so they could tell me whether it was even worth my drive out.
The other confirmation is visual. Cavitation almost always shows turbulence or a churning, foamy look through the clear pump lid — you can watch the water boiling where it should be smooth. A pump with a genuine bearing or clogged impeller problem runs with smooth-looking water in the lid. Match the sound to what you see and you’ve got your answer.
Stop It From Coming Back
Cavitation is a maintenance problem more than a repair problem. Keep the water at mid-skimmer — check it weekly in summer when evaporation and splash-out drop it fast. On my own pad I check the water line every Sunday when I clean the baskets — it takes me thirty seconds and it’s saved me this exact headache more than once. Clean both baskets on a schedule instead of waiting for a symptom. And if you’re shopping for a new pump, size it to your plumbing, not to the biggest number on the shelf. A right-sized variable-speed pump run at a moderate RPM almost never cavitates because it isn’t demanding more than the pipe can give. Replacement lid o-rings and gaskets run $8–20 — you can grab them at a pool supply store or on Inyo Pools by searching your pump model.
One quick note on cause-and-effect, because cavitation overlaps with a couple other problems: if your pump is also struggling to hold prime, the same suction-side air leak can be behind both — see the grinding noise guide if the sound is more of a metal-on-metal grind than a gravel rattle, because that’s bearings, not cavitation.
Will Cavitation Damage My Pool Pump?
Yes, if you let it run. Short-term, a few hours of cavitation while you track down a low water level won’t hurt anything. But weeks or months of it pits the impeller vanes, wears the shaft seal, and drops your flow and pressure as the impeller loses its edge. I’ve pulled impellers that looked sandblasted from a season of ignored cavitation. The damage is gradual, which is exactly why people ignore it — it doesn’t fail today, it fails in a year. Treat the gravel sound as a do-it-now problem and you’ll never replace an impeller over it.
Frequently Asked Questions
What does pool pump cavitation sound like?
Like the pump is full of gravel, marbles, or rocks — a rattling, sucking, churning sound that rises and falls with flow. It’s different from a steady bearing growl or a high-pitched screech. If the sound changes when you restrict the skimmer, it’s cavitation.
Is cavitation the same thing as an air leak?
Not quite — an air leak is one of the causes of cavitation, not the whole story. A pump can cavitate from low water or a clogged basket with zero air leak. But every suction-side air leak will cause cavitation, so they show up together a lot. I always check for air leaks as part of a cavitation diagnosis even when the water level looks fine.
Can a variable-speed pump cavitate?
Yes, especially at high RPM where it’s demanding the most water. The nice part is that dropping the speed is an instant test and often an instant fix — lower RPM means less suction demand and less chance of starving the line. If your variable-speed pump only cavitates at full speed, that’s telling you the plumbing can’t feed it at that rate.
Will adding water really stop the noise?
If the water was below mid-skimmer, often yes — and fast. I’ve stood at pumps that sounded like they were dying, added water for two minutes, and watched the noise disappear completely. Low water is the single most common cause, and the hose is the cheapest tool you own. Always rule it out first. It’s the first thing I check on my own pool whenever a pump starts sounding rough.
How long can a pump run while cavitating before it’s damaged?
Hours are fine; weeks and months are not. There’s no exact line, but the practical answer is fix it within a day or two and you won’t see meaningful damage. Let it cavitate all season and you’ll be looking at a pitted impeller and possibly a worn shaft seal. The cost of ignoring it is gradual but real.
Related Pool Pump Guides
- Pool Pump Grinding Noise — What It Means and How to Fix It
- Pool Pump Impeller Clogged — How to Diagnose and Clear It Yourself
- Pool Pump Making Noise — What Each Sound Means and How to Fix It
- Pool Pump Not Priming — 7 Causes and How to Fix Each One
- Pool Pump Strainer Basket Problems — Not Sealing, Cracked, and Clogged