The whistling sound a pressure cooker makes while it works comes from pressurised steam vibrating as it escapes through the valve on the lid. The sound is not a warning; it means the cooker has reached its working pressure and cooking has properly begun, which is usually the point where the heat is turned down and the timer starts.
Why does the sound come in bursts?
Most pressure cookers have a regulator sitting on top of the lid. Once the steam inside crosses a threshold, this piece lifts slightly and releases the excess, then settles back down as the pressure drops. That rise-and-fall cycle is why the sound is intermittent rather than continuous. If the heat underneath is too low, pressure never builds enough to lift the valve; if it is too high, the valve stays open and the sound turns into a constant hiss that wastes energy and boils away the liquid inside too quickly.
Older weighted-valve cookers tend to produce a sharper, longer whistle, while newer gasket-sealed models give a shorter sound closer to a hiss. Both work on the same physical principle; the difference is purely in the valve's mechanical design. Weighted valves usually sound louder because steam escapes through one narrow point, while gasket-sealed models let steam spread along the rim, giving a softer, steadier sound.
What does it mean if there is no sound, or if it is very loud?
If nothing happens for a long time after closing the lid, there usually is not enough liquid inside or the gasket has not seated properly, so pressure never builds and the food cooks as slowly as it would in an ordinary pot. On the other hand, a constant strong hiss from the lid usually means the cooker is overfilled or the valve is not sitting correctly, which calls for turning the heat down and checking the lid.
For safe use, the cooker should be filled to at most two-thirds of its capacity, or half for foaming ingredients like legumes. Without enough liquid, the sound cannot form and the risk of scorching the base rises. A gasket that has gone stiff or cracked gives the same symptom; replacing it usually restores the cooker's ability to hold pressure properly.
In practice, learning to recognise the sound is the most reliable way to time a dish correctly: once the steady, intermittent hiss begins, that is usually the moment the timer starts counting. Anyone cooking the same dish in different brands of cooker should judge by this steady rhythm rather than the loudness, since the moment pressure settles always shows up as this rhythmic sound regardless of make or model.
Altitude also affects how soon the sound appears; in a kitchen close to sea level, water boils at a higher temperature so pressure builds faster, while at high altitude, where water boils at a lower temperature, the whistle can take a little longer to start. How the heat is distributed underneath matters too; on ceramic or induction stovetops, heat spreads faster and more evenly, so the whistle can start a little sooner than it would on a gas burner.