Last night I was out messing with the Regal, and I couldn't get it to start. When I first tried to start it, all I heard was a clunk noise. I turned the by hand and it would turn completely, full rotation, and sometimes it would get stuck, and you would hear is a clunk. When I tried turning the ignition on, the car almost started, but backfired through the exhaust and didn't come on. I'm in the process of removing the timing cover, but what do you guys think it can be?