Ford 2012 Expedition 5.4 Engine is knocking!

Hello, I am not aware of any drive through car wash that could inspire an engine to start knocking. As far as "blowing a plug" - is there a spark plug out of the engine? That would be more of a loud popping noise from the engine when running, more than a "knocking", and would be visually noticeable under the hood. If this was me, I would be lobbying pretty hard and heavy at this Lexus dealership to make things right for you. I do not suspect they "knew" of any pre-existing issue, but it would be highly unusual and coincidental (yet possible) for a failure to occur after only 5 days. Also, an evaluation to determine the source of noise is a good idea to formulate a theory as to whether some kind of instant catastrophic failure occurred, or if something has been deteriorating over a longer period of time. Lastly, a close evaluation of the oil and filter change likely performed during vehicle reconditioning and inspection is a good idea. If the filter is not proper for the vehicle, has debris inside of it, or the improper oil level or grade was used, this could influence what is happening. Good luck!

Have a qualified mechanic *listen* to this "knocking" sound to determine exactly what the actual source is .. It's quite possibly something to do with the drive belt or the belt driven accessories after the car wash deal... Sometimes spraying the drive belt circuit with water - or other fluids that were in the wash soulition - will cause all kind of weard noises - for a pretty good while! Good luck with it!