My suspicion is that there is something loose (pebble, rock, or some other small, foreign object) behind the hub cap, but I hear a crackling or popping sound (almost like Pop Rocks or Rice Krispies) when my 2011 Kia Rio gently rolls forward or backward at a slow speed. It almost sounds "sandy," if that makes sense, like the noise a heavy car might make when rolling forward over a sandy or salted pavement. (I'm trying to be as descriptive as possible to help cue people in.) However, it seems to occur even on smooth pavement, and the Kia Rio is comparatively not a heavy car.
The sound either goes away or becomes inaudible (indistinguishable from background noise) when actually driving. It does not occur when immobile/idling. The sound is irregular, in the sense that it seems more like random crackling or popping. From the driver's seat, it seems like it's coming from beneath the car or near the wheels.
This is my first car in many years--I have lived in a city and been using public transit for a long time--so I am willing to admit that I am just not accustomed to the sounds that a car normally makes. Preliminary Internet research seems to have ruled out things like bad brakes or suspension, but I'm not sure. It would be great to learn that this is just one of those noises cars can make, and that it's innocuous.