A loud banging or clanking noise occurring right after a sump pump shuts off is typically caused by water hammer resulting from a standard swing check valve slamming shut violently. When the pump motor turns off, the column of water remaining in the vertical discharge pipe crashes downward under gravity against the check valve flapper, creating a loud shockwave through your plumbing pipes. Verification: Replace the standard swing check valve with a modern spring-loaded check valve designed to absorb shock and close silently.
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