I agree with Saoltmann. Its most likely expansion and contraction. PVC and ABS piping expand and contract substantially in the walls when hot water is run through the piping. Especially water comming from a dishwasher. Many times no consideration was made for the expansion and contraction in the holes through the wood. So when the pipe expands (about 3" in 100' or parts thereof) it rubs against the wood. You may be able to solve it, but it will be somewhat costly for the drywall repair.