Hello - I have a perplexing problem. The past month or so I've noticed a distinct dripping sound in two places whenever I use any of the plumbing in my two-story townhouse. When I run the kitchen sink (w/ hot or cold water), I hear a dripping sound in the ceiling above the kitchen (and the sound quickly disappears) and more distinctly, a dripping sound in the first-floor bathroom wall (the common wall with my neighbor). When I flush the toilets upstairs or use any of the plumbing upstairs, I've run downstairs and the dripping sound in that wall is very loud and steady while, for example, the toilet is running. Once the toilet stops running, the sound immediately ceases but then I hear a drip... drip... drip at irregular intervals for a very long time afterwards. I've put my ear to the wall to try to decipher where in the wall its coming from, but I can't be sure. It's pretty loud though. Any help would be appreciated. Thanks!