I have s funky smell in my guest room when using the shower in the bathroom next door. No water marks anywhere. Had plber to see what's going on he said it might be from the drain, since the smell is located in that area... ( I'm smelling the wall) he said, put extra insulation in wall through contacts and use liquids to clear the drain twice. The smell is still there.
What can it be and how can I get rid of that smell..?