Hmmm...Did you drive through any deep water recently? I have replaced several GA16DE's that ingested water, and hydra-locked. That means water got into the cylinder [water doesn't compress like air does], and when the piston came up, it couldn't compress the water, and bent the connecting rod. All three of the customer's engines I replaced, had a bent rod. The noise came from the side of the bent rod, as it hammered the bottom of the cylinder wall each time it passed. Other than that, it ran fine. I actually benefited from this, as I got to keep all of the old engines, and fix them. I used two good rod and piston assys from the bad engine, to repair the other two. I honed the cyliders of both, and reassembled them So, for the price of two head gaskets and a tube of gasket maker [for the oil pans], I had two good GA16DE's at my disposal.