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