A sharp knock just as the washing machine stops filling can come from water hammer. The washer closes its inlet valve quickly, and the moving water sends a pressure wave through the supply line. A loose pipe can make the noise louder when it moves against framing.
Notice when the knock happens
Listen during a normal cycle. A knock at the end of filling points toward the supply side. Noise when the washer pumps out may involve the drain or the appliance itself. Note whether the sound occurs on hot fills, cold fills, or both, and whether another fast-closing fixture causes it.
Ask the plumber to check pressure and support
Share a recording and describe where the sound travels. A plumber can check supply pressure, pipe support, and whether an arrestor near the washer would help. If a hose or connection is visibly leaking, stop using the washer and arrange service. See our home plumbing service for help assessing supply lines and connections.