So this is for my friend who has a 2003 MCS with the JCW kit dealer installed. After a day of backroad driving, we were on our way back to his house and all of a sudden all the lights on the dash light up and there is this awful knocking noise from the engine.

The chances of it being a connecting rod are slim since this did not happen under any sort of load, and the engine had oil. Happened at about 2K rpms and 45 mph while just driving normally. Checked balancer and that is fine.

Could it be a rod knock, or what about a failed lifter? Are either of these common? Also checked tensioner by pulling valve cover and seemed fine.

I can post a video if needed.

 

A CEL and loud knocking from the engine are grounds for a rebuild.

Failed lifter would be more of a tick to a clatter, not a knock.