Made a thread about this a few weeks ago, and think I may have found the culprit.
One morning I had this odd clicking/clapping sound coming from the drivers side of the engine. Video of it
here.Scoped it as best I knew how, and all I could gather was that it was indeed coming from the drivers side. Parked it for a few weeks and drove my other car while I pondered it over and waited to find a post with similar symptoms.
Then, while helping a coworker with his BMW 5 series suspension refresh, I decided to start it and take another hack at with the scope. Started it up and the sound was gone! At this point I thought it had to be an injector.
Been driving it the past few weeks with little to no issue. The knocking sound came back one time then went away when I have it a little gas. Fast forward to tonight and I'm getting on the interstate when boom. Running like shit, making a god awful sound that was similar to a belt about to go. Pulled over and called a friend that I had just left dinner with to come get me.
We're poking around when he notices that cylinder 4 (middle on the drivers side) has its spark wire off. I think to myself
what in the world and go to put it back in but its not going on right. Shine a flash light down there and see the plug is completely unseated and loosely rolling around. Try is we might, in that tight space, we couldnt fish it out to get a better look. So my friend cupped his hand over the hole and I cranked to see if we could get the plug to change its orientation. Well, it shot out and bounced around the engine bay.
Neither of us had a spark plug socket on us so we went to the auto parts store and grabbed one. Screwed it, being super careful to feel for the threads and make sure they were intact and were blown out. Got it in, snugged it up, still cautions it was the threads that gave, put the wire on and its running as good as new.
Dealt with many things, a spark plug backing all the way out is a new one for me.