I hear a drip inside the wall for about 5 minutes after tub or sink used not when toilet is flushed. Plumber and handy men think its just an echo from last dribbles of H2O in PVC pipe ;unheard in or castiron pipes. No water noted in basement at sewer line out.Had line replaced 10 yrs ago. Thanks michaele

5 minutes seems a bit long for a drip but could be. Noise is always more prevalent in PVC then Cast Iron. Could also be an expansion and contraction noise from the PVC. Try this, use only cold water and then drain. Still hear the noise? Now try with hot water. Hear the noise? If only with hot water, the noise is from the PVC moving through the studs.