Wheel hubs bad?

I recently noticed that when spinning the front tires on my 2002 Tahoe 2WD while on jack stands, I noticed a slight clicking type noise that I believe is coming from the wheel hubs. It sounds like little pebbles are inside. I figured the bearings inside the hub were going. The truck has 46,000 miles on it. It does this on both sides. However, when driving the vehicle, there is no indication of a problem. It still rides as smooth as the day we bought it new. Could it be the ABS sensors? Anyone else have this problem? Any suggestions? Should I replace them? I find it odd that both sides would be bad at the same time. Thanks for any advice.