Was a cold morning (15F) when I went to start the car and although it started right up it was lugging/idling terribly rough and suddenly started knocking loudly, much like a diesel engine. I shut it right down and waited a few seconds to start it again. It started/idled smoothly but the knocking persisted. Slowly after about 5 to 10 minutes it got quieter and mostly went away. No problems for at least 2 months. Then, on another cool morning (30F), started up car and again lugging/idling terribly, grey smoke smelling like unburned gas, and diesel-like knocking with an intermittent, louder, "CLANG" sound.

For now I'm at a loss as to what it could be. I'm not going to run it again till I find the problem because it sounds pretty serious. Any ideas? I have some audio and video files of the sound if anyone is interested. Thanks.

Oil level was fine, always did my own oil changes (synthetic every 4-5k miles).