DIAGNOSIS That's the pressure regulator (the weighted valve) "jiggling" excessively. It's normal for it to rock and hiss a bit to regulate pressure, but loud rattling means your heat is too high.
FIX As soon as the cooker reaches pressure (the float valve pops up) and the regulator starts rocking, immediately turn down the heat on your stove. You want to lower it just enough so the regulator maintains a steady, gentle rocking or hissing—maybe 2-4 times a minute—not a frantic chatter.
IF THAT DIDN'T WORK Make sure you're using the correct weight for the pressure you need (e.g., 15 psi weight for high pressure). Also check that the valve is seated cleanly on its post and isn't physically obstructed by a splatter shield or a too-tall steamer basket inside.
HEADS UP That loud noise means pressure is spiking and releasing rapidly, which can lead to foaming or food getting sucked into the valve. It's not immediately dangerous due to the safety systems, but it's bad for cooking and for the pot's internal components over time.