So my roommate owns a 2014-2016 Hyundai Elantra (Don't know the year, most likely a 2015).
It's an absolute enigma. All of the doors are different colors underneath the paint. (In fact every body panel is a different color)
There is evidence of a frame puller having been used.
The trunk floor looks like it's been welded together.
The VIN returns 3-4 different vehicles. (depends on which service you use)
The interior has pieces from different Elantra trims and most of the pieces do not work. (The horn doesn't even work (well it doesn't even have one))
It's dropped the oil pan twice now while driving.
It's lost an oil filter while driving
Running in limp mode and will not rev above 5,000 RPM. It gets there and blows thick white smoke out of the tailpipe. (However no speed limiter)
A rat ate most of the wiring at some point.
It has some wiring issues. Sometimes the dash lights just turn on and then go away after a little bit. Or the turn signals suddenly get funky and go super fast or will just stay illuminated and never blink. Depends on the day.

Just an utter mystery of a car. Not a salvage title believe it or not.

Anyways this morning we started the nugget and it started fine, and after about 30 seconds I noticed the engine sounded like it was tapping, then knocking. And continued to do so. Oil level is fine, a little low but not near minimum. A little old but nothing bad still had some color and the viscosity seemed alright.
This is a new issue. Aside from the times where it lost all oil it has not knocked before.
Anyone know what causes a knocking sound?
And how the hell does a car like this even come about? Dirty Dealership? Stolen then sold?