There is a slight smell of dampness in the bathroom, and the water in the sink drains slowly. Could this be due to a blockage in the sewage system? Attempts to clear it with chemicals and a plunger have not helped. I want to understand if the problem is further down the pipe and how to safely fix it so that I don't have to replace the entire system and create even bigger problems in the house.

After recently cleaning the sink trap in the kitchen, the water still drains slowly. It seems like there is a more serious problem with the pipes, perhaps the drainage or sewage system is clogged at a deeper level. I don't want to blindly dismantle the entire structure and make things worse. I need to find a safe way to check where the problem is without risking damage to the system.