I'm copying this from another forum:

I hope that's not the case but it didn't sound good. So, yesterday I went for a nice 44 mile round trip drive and the car was running great. Today I did some minor work on the electrical system (Installed a new alternator wire) and removed and rearranged some spark plug wires while I was at it... I went to start the engine and it cranked right up but it was knocking pretty loud and shaking...Almost sounded like it was backfiring or a piston hitting metal at the top of its stroke. I immediately shut it off and assumed that maybe I had installed the spark plug wires incorrectly, but the wires were fine