Got a strange one here. In the past year, one section of my ~13 year old
concrete driveway (roughly 10' square) has developed a couple of cracks and
settled 1/4" - 1/2" in the center relative to the edges. By itself, that
wasn't enough to really bother me (other than a "that's weird"), but this
weekend I noticed that section actually had a noticeable "drumming" sound
compared to the other parts of the driveway when I dribbled a basketball on it.
Not only did that make me lose concentration during a cutthroat game of "HORSE",
it also made me start worrying a bit.

Anyhow, there must be a bit of a hollow under the driveway in that area. No
water, sewer, or sprinkler lines in the vicinity, so I don't think it's water
erosion. Not sure how to go about addressing the problem, 'tho. Just get a
mud-jacker out and cross my fingers, or are there other things that need to be
investigated first?