Last edited by mcbachma; 11-24-12 at 01:58 PM.
You should be able to find it by inspecting it. Spin the wheel and rub your finger along the rim while it spins.
If it's minor, you might be able to bang it back in shape with a hammer. You can do this on a steel rim. Or if it's really small, it might be a burr which you can file down.
See if you can post a picture.
It might not be a blip; it might be a low spot, though I'd hope your bike shop would notice that!
The outside-width of the rim may also vary at different spots on the rim (due to hitting pot-holes and kerbs in the past). Take the tyre off the rim and measure the outside-width with some vernier calipers. Then slide the caliper around the entire circumference of the rim and it may get stuck at a spot that's wider than the rest. I've squeezed these wide spots back to parallel by clamping the rim inside a vice. Do a little at a time and re-measure with the calipers.