The wife's '21 Flow had a rattle that made ME nuts while riding next to her! Big DIY'er here, so I looked for that rattle to the point I was getting pretty frustrated with it....
Finally, after a couple of weeks of listening to this rattle and long after going over the list of potential suspects causing it several times, I was riding next to her one day and happened to be looking at the kickstand when the rattle occurred. An expansion joint in the pavement would cause the kickstand to drop down 1/2" or so, then it would return to it's normal position with a rattle. Just enough noise to be an irritation to somebody with an otherwise silent bike. Fix was to shim the spring with a washer, adding just enough tension where the spring could hold the kickstand up when hitting a bump.