hi, i have a salty residue which have appeared inside my fireplace and also on one of my walls and also the plaster sounds hollow if tapped. both the fire place and the wall are on internal parts of my house. is this damp???

Possibly efflorescence. Remember that damp can come down the chimney or up from the ground.

Is the wall in question a stud partition? Or is the wall plastered on the hard -- i.e. plastered directly on to the stone or brick or block?

John

hi, its of a brick wall that is plastered. and its a internal wall.