If you only fitted the machine heads loosely, and didn't then tighten up the clamping nuts fully, the washer can still vibrate.

If you fitted all the tuners but only two strings, then the posts can rattle against the clamping nuts.

If this is the ST kit, then the top E won't have its string tree fitted yet, so the string can buzz in the nut slot due to lack of downward pressure.

And any string can buzz in its nut slot if the slot is cut/angled badly so the string is not resting firmly at the fingerboard end but at the headstock end of the nut so the string can vibrate against the nut.

You should also check the truss rod. The kit have double action rods, which have a dead spot in the middle between adding tension or compression to the rod. The truss rod nut is loose in this mid-point, so can sometimes rattle. Just turn it enough so it starts to bite (either way), and see if that stops any rattle.