2001 Ford Taurus SE - Engine Knock

We bought my son a 2001 Ford Taurus SE with 115k miles, Automatic, 6 cylinder engine. He was driving home when the engine started to make a knocking noise and the engine light came on. When I started it there was a loud knocking/pounding noise coming from the engine. Oil level was fine. Since I am not mechanically inclined I have no idea what is the cause and am looking for some guidance before taking to repair shop.

Any suggestions / ideas? Cost estimates?

Any help would be appreciated.