When water is being drained from the upstairs bathtub or shower (different locations)...I can hear a drip-drip-drip sound behind the kitchen wall (seeming to get faster all the time). When asking my husband, he said it is just the water from the second floor hitting the pipe on the way down...?? Not that I don't believe him.... :-) but Bob Villa he is not. Our house is 15 years old and I don't recall this sound before. What is the easiest way to tell if this really is a leak somewhere or just water "falling".....???