What causes the sudden spool-up sound from an F-16 when enabling afterburner? What causes the sudden spool-up sound from an F-16 when enabling afterburner?

I've included a video to make it clear what sound I'm referring to:


                
            

at 1:16, I assume the pilot engages afterburner (again), and there is a sudden spool-up sound associated with this.

Am I even correct in assuming this is what's happening? I don't understand what this sound is though, it doesn't seem to appear consistently. Throughout the video, it appears the pilot engages afterburner and turns it off multiple times, but the sound only appears once.

Bonus question: What is the purpose of running the engine like this, and how is the F-16 attached to the ground?
My first guess based on the sound repeating when the thrust is idled near the end of the video is that's the sound of the actuators of the variable geometry nozzle.

Googling that actually auto-completed it to Great F-16 Actuator Sound (video).

Bonus answers to the bonus questions:


It's a ground test of the engine, making sure it's working fine with no leaks.
The F-16 has a tail-hook like carrier-borne planes, if you look closely a cable is attached to that hook, which itself is attached to the ground. The F-16, F-15, and other land-based fighters have it in case of an emergency, they'd land on a runway with a braking wire across it.