There are two components in PAX that will make a bit of noise.
The mouthpiece has a small, sliding ring on the metal stem. This piece needs to freely shake back and forth for the mouthpiece to open and close properly - keep this clean to prevent your mouthpiece from getting jammed with build-up.
The other part you'll hear is the internal accelerometer, a loose ball bearing with sensors around it that detects the motion of PAX. When you shake the device to check the battery level, you'll hear the accelerometer making a slight clicking noise.