I can almost guarantee you it is in fact the inserts. It almost sounds like dirt is being flung up against them but not possible when going 1 mph. I know its sounds odd and I wouldn't think the wheels are flexing but not really sure of any other explanation other than the fittings being super tight and there being some flex when moving around. Someone else noticed it on the thread I made as well so not just me.
Here is a vid of the sound. Watch IMG_7655 | Streamable
also its not only one insert, sounds like its coming from all 4 wheels
Having taken the wheels off mine and removed the inserts, I don't see how they could flex or click. They are super tight. Have you gone around the car and tried to jiggle each one (even the small ones) by hand to see if any are loose? Also take off and reinstall the center caps too: I don't know how but what if there's a small pebble or some sort of debris inside the center cap(s)?