Rough rim seam. The spot where the rim was joined by the manufacturer (Bontranger?). Check the spot in the rim directly oppisite of the valve stem and feel for a raised bump the height of the braking surface.
OR, you could have a dent in the braking surface of the rim. To find this, just feel around the rim until you find a depression in the braking surface.
This is assuming the wheel is true when you stated the wheel has no side to side movemnt.