The sink and tub drain pipes for both second floor bathrooms seem to connect and run down inside the wall between the kitchen and dinning room. When water is drained you can hear it flowing down the pipes in the dinning room. My concern is that when the flow stops you may also hear a dripping sound for another several seconds. There are no wall stains to indicate a hole in the pipe. The house is 18 years old. Is there a problem that needs a plumber.

Thanks