Diagnosis
That's the internal pressure switch (a magnetic relay) cycling on and off rapidly. It's trying to maintain temperature. Excessive clicking usually means insufficient liquid.
How to Fix It
For pressure cooking, you must have at least 1/2 cup (125ml) of thin liquid (water, broth) in the pot. Ensure you're not using a thick sauce or paste as your only liquid. Add more water.
If That Didn't Work
The inner pot may not be sitting flat on the heating element. Remove it and check for food debris or moisture on the bottom of the pot or inside the cooker base. Wipe both clean and dry.
Heads Up
Continuous rapid clicking can overstress the switch. If adding liquid doesn't stop it within a minute, cancel the cycle. Let it cool, check the pot position, and restart.