Diagnosis
One of the two lift columns is failing. The grinding is metal-on-metal wear inside the telescoping leg, likely from a damaged gear or lack of grease.
How to Fix It
Stop using the desk immediately. This will get worse. Contact SHW support directly. You will need a replacement lift column assembly. They are sold and installed as a matched pair for safety and synchronization.
If That Didn't Work
Before assuming the column is dead, check for a foreign object. Unplug the desk and inspect the full length of the noisy leg's outer tube. Look for a small screw, staple, or piece of debris lodged in the seam.
Heads Up
Do not attempt to lubricate the column yourself. Most consumer lubricants will attract dust and make the problem worse. The internal mechanism is sealed.