There's likely mold growing behind that drywall near the plumbing—probably from a slow leak or condensation that's been going on for a while. The musty smell is a dead giveaway, and if the wall feels soft or shows any dark stains, it's time to open it up and see what's really going on. Ignoring it usually makes the problem spread and get more expensive to fix. mold behind drywall near plumbing 51