The bang or thud in pipes when fast-closing valves (washing machines, dishwashers) slam moving water to a stop. Arrestors absorb the shock; chronic hammering also points at excess pressure worth checking.
Plumbing terminology in MD, DC, and Northern Virginia maps to older supply lines, basement sump pumps, water pressure regulation, and the licensing (MD PLB & GFC, VA, DC) that real plumbing work requires.