DIAGNOSIS The piston or striker inside the hammer cylinder is damaged. A common failure is the striker cracking or the retaining ring coming loose.
FIX This requires disassembly. Unplug tool. Remove the gear housing. Tip the hammer cylinder out. Inspect the internal piston and the striker (the free-moving steel piece inside the cylinder). Look for cracks or chips. The striker is part number 1 607 001 006. Replace if damaged.
IF THAT DIDN'T WORK The bearings on the wobble plate or drive shaft could be shot, causing metal-on-metal contact. You'll need to replace the entire crank assembly.
HEADS UP Running the tool with a broken striker can fragment the hammer cylinder. Stop using it immediately when you hear this abnormal metallic clank.