You either have a "flat spot" in your rim, or a bead-seating issue with your tire. Look very carefully to see if the rim's radius changes slightly at one point. Spin it in the bike fast, and look at one point to watch as the rim travels through your view. If it goes in towards the hub, it's a flat spot. If not, you might try deflating your tire, and wiggling it left and right all the way around. Make sure no tube is coming between the bead and the rim sidewall.