Be warned that I’m not sure the below presented solution fixed this! I suspect it was because the top nut, that screws the top plate of the air bag to the damper rod, was undone. Sounds exactly like a metal plate rubbing on a screw thread. As shown in the photos, you can check it without taking the strut out.
Take out the air tube, then unscrew the 3 strut nuts that hold the strut to the chassy. You need to do this to allow clearance for popping out the top plate from the underside. Slip your hand between the chassis flange and the strut, as there is no clearance to extract it upwards through the hole.
Now you can inspect the internal nut. The rod has an inside hexagon for keeping it in place while tightening. Because there was no room to do this as shown / I did not had the proper tools, I tightened it with an impact tool, so there is less chance for the rod to rotate.
For more insight, here are some photos from when I replaced the airbag on the strut.