Help with fixing smell in house. It is either just the smell of the attic or the slightest plumbing leak?

So, my basement and sometimes my two bathrooms which are back to back smell. It is not horrible, like sewer gas when you're replacing a wax seal, but it is noticeable. The main waste stack is located between the two bathrooms. From the basement, I can look up to the first floor and I do not see any visible leaks and again, whatever it is doesn't smell like sewer gas. From the attic, in between the walls, I can see down to the floor of the first floor and again, I do not see any leaks or notice a sewer gas smell. The attic smells the same as the basement and bathrooms. It may just be the smell of the attic getting down in between the walls and traveling down to the basement. The main waste stack has about 1/2 gap in between the pipe and the floor. Also, in the attack, aside from insulation, there is no covering on this space in between the walls. So, should I seal up the area near the waste stack with silicone or insulation? Also, should I put a plywood cover over this gap between the walls to keep the attic smell out? Or finally, should I investigate the soil pipe (main waste stack) thoroughly. It is cast iron with joints sealed with lead and all appears to be in good shape. The house is about 60 years old.