I have a 2013 Subaru Impreza with a 2.0 liter engine and a manual transmission, and when I let out the clutch too fast while in reverse or 1st, sometimes the engine RPM will drop so low that it makes a nightmarish knocking sound before recovering. What is that dreadful noise? ( is it the transmission? the engine mounts? the piston grinding away at something god forbid? )
I'm familiar with the knocking caused by air being too hot and/or the fuel mixture being too lean, but to the best of my knowledge that happens under power and usually not with the engine is simply idling.