Raven
Just a thought – try pouring a quantity of water directly down the shower waste pipe and then listen for the noise you are hearing – if it is there then it may just be as said above the water dribbling down inside a plastic waste pipe within the wall – or it could even be that the shower waste pipe is connected into the same waste pipe that the kitchen sink runs into and the sound is just an amplification of water running through a plastic waste pipe.
You do not say if you have a concrete floor or what so I suggest that if it is wooden with a crawl space under it have someone run the shower while you look round the floor plate for any wet or water dripping down – this would obviously indicate a leak is within the wall cavity somewhere.
Hope this helps some.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians