The biggest single cause of low speed rim ticking is small bits free to move around inside the rim. Often when rims are drilled, small chips fall into the rim. Makers shake them as best they can, and they fall out on their own if the rim has no eyelets. But if the maker doesn't get them all before installing the double eyelets, they're stuck there forever.

Some also don't shake out because they're still attached to the inside lip of the hole as large internal burrs. They'll stay in place for a while and eventually break off and cause rattles. The typical rattle caused by these only happens at low speeds, al they flop around like clothes in a front load washer. With higher speed, the wheel is in spin cycle and they're stuck in place on the outer wall.

Of course there might be something else going on, but if the wheel is aligned, and the braking in ever, indicating no variation in width, then just ride it and don't fret. If it really bothers you (it shouldn't) you can try to localize the area in the rim, and either see if you can work whatever it is down to the valve hole and out, or drill a small hole through the inner (sire) side of the rim, and inject a blob of grease or glue and try to trap it.