I have a new home on a slab foundation has a mildew smelling wall section about 5 to 8 feet above the floor on both sides of the interior wall. The builder opened one side of the wall near the floor by the sink plumbing and vent....no leaks were sighted. The builder then removed a one foot by one foot section of drywall about 5 feet from the floor and then got a strong pungent odor when the drywall was removed. We saw on mold or evidence of any leaks, but now 3 months later the odor has resumed and even worse. Some days the odor is very strong and Hits you when you enter the bedroom, other days, almost non existent. Also, we checked the second story for evidence of leaks and found no staining. A hot water heater is located above this area but at least 5 feet away from the odorous area and no evidence of leaks again. Does anybody have an idea what this smell could be? Could this be related to a past leak during construction or a vent pipe issue? Any imput or suggestions would be appreciated on how to resolve this mystery.