Diagnosis
Debris is inside the telescoping leg column or the gearbox is failing. Metal shavings or dirt are the most common cause.
How to Fix It
Lower the desk completely. Use a vacuum with a hose attachment to clean around the leg seam where it telescopes. Extend and retract the desk a few times while vacuuming to draw out particles.
If That Didn't Work
The noise is likely internal. For a grinding sound, the helical gears in the gearbox are wearing. For a rhythmic click, a tooth may be chipped. The motor/gearbox assembly needs replacement.
Heads Up
Continuing to use a grinding desk will destroy the gearbox. The repair requires removing the desk top and frame to access the motor mounts.