Years ago, a customer brought in a 380SE with a knock. I traced the noise down to the block, and pulled the right side head. It took me a while to find that #8 piston was not moving all the way up in the cylinder. The intake manifold gasket had broken and allowed coolant to enter the cylinder causing a hydraulic lock that bent the connecting rod. The knock was the piston kissing the crankshaft at the bottom of the stroke.