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?

If you've lost all the oil twice before then no doubt there is some bearing damage that is starting to make itself known. When you lose all oil suddenly the main bearings usually suffer, when you run it low over a period of time the rod bearings suffer first. Yours probably had some main bearing damage that has progressed to the rod bearings as the mains feed the rods their oil. It will fail soon.

Ouch. I don't doubt it going soon either. Some of those cold starts are horrifying.
Got any ideas on how to milk the rest of the life out of this engine?

If in fact it's a rod bearing failing when it goes it will probably go big, ruin crank for sure and lock up, or worse put the rod through the block. If it were mine I would quit driving it and have the pan removed for inspection, you "might" be able to save it with new bearings if crank not already damaged beyond use. An "in chassis" repair can be accomplished on some vehicles depending on accessibility, been some youtubes on it.

It's been a couple of months now, what the verdict?

Another thing that could cause the knocking is a bad oil pump, did/does it have oil pressure?

My first thought is why would anyone keep this car, put themselves or others at risk driving it?

Stopped knocking. Don't know why, but it just stopped. Nothing added nothing changed. On cold starts every now and again it knocks, but since it's getting warmer I haven't heard it.
Engine is still in limp mode. Cannot safely go above 5K RPM. When it does it blows thick white smoke from the rear.

We also found that the title he has is not the original title. It's a copy of the brand new title. Because somehow the dealership "Lost the title" and coincidentally there just happens to be frame puller evidence and most of the front end is from a different car, and there's two vin numbers, the engine is not the correct one, the list goes on. Shady AF dealership.

What did happen is that the transmission just kinda
exploded
Thing wouldn't leave fourth gear and was bogging really, really bad. The last thing it did was full throttle reversing into a parking spot. There was a trail across Lincoln NE of trans fluid.
We looked at the transmission, but i really seems like you weren't ever supposed to fix it. Completely sealed no trans fluid stick, no way to see it.

teenagers and hand me down vehicles
half of the reason i still have my 4runner with all of its issues. (The other half is that I love the car too much to let it go.)