A loud bang or shudder through the pipes when the pump kicks on or a faucet shuts off isn't just annoying — over time it can loosen fittings and shorten equipment life.
Water hammer happens when a moving column of water is suddenly stopped — usually when a fast-closing valve or faucet shuts, or when the pump starts and slams water into a closed system. That sudden stop converts the water's momentum into a pressure spike that travels through the pipes and hits fittings, walls, and the pressure tank hard enough to hear and sometimes feel.
Quick check: If the banging happens specifically when the pump starts (not when faucets shut off), check your pressure tank's air charge first — a waterlogged tank is the single most common well-system cause of water hammer.
Start with the cheapest fix: confirm your pressure tank has the correct air pre-charge (2 PSI below the pump's cut-in pressure, checked with the tank drained and power off). If the tank is healthy and the hammer persists, a water hammer arrestor — a small sealed air chamber installed near the noisiest fixture or at the main line — absorbs the shock instead of letting it travel through the whole system.
Persistent water hammer that isn't fixed by a healthy tank and an arrestor can indicate pipes that are poorly secured (loose straps let pipes move and bang against framing) or a system that's genuinely oversized for its plumbing. Both are worth a plumber's inspection, since repeated hammer over years can loosen solder joints and shorten the life of valves and fittings throughout the house.