Originally Posted by
FBinNY
If it's a single place, and at the same place on both sides, odds are it's the remnant of the weld. (obviously only applies if it's a welded rim).
After welding the rim has the sidewalls (brake tracks) machined, but most makers only do a very cursory job on the insides, using a grinder. Since it's a complex shape, perfection isn't possible, so they get it decent and leave it at that.
As long as there aren't sharp edges, feel free to consider it as a belly button.
In both rims, it IS near the join points, so that makes sense. I suppose "sharp edges" would be exaggerating the feel, but they aren't perfectly smooth by any means.