1999 Ford Taurus-3.0L V6- Knocking Noise

1999 Ford Taurus, 3.0L V6, 158000 Miles

I bought this car from a friend who had said originally that her sister ran the car through a low water crossing and blowed the motor. I thought she just meant her sister had splashed through the crossing too fast. I am going to call her this evening b/c when picking up the car yesterday she made the comment that they took the air filter out b/c it was wet due to the water. I got to thinking, that must mean the car was underwater?

She is not a car person and took the car to the mechanic and he told her it was the "rods" and would cost $1200 to fix. She took this to mean the "tie rods". I did not know that he had told her "rods" and she interpeted as "tie rods". Anyhow, she had already bought a new car and was just going to sell to salvage yard and I bought it for $200.

I went and picked up car last night. We started it and it made NO noise whatsoever and we drove up on the trailer and hauled home.

This morning, we unloaded it and it is knocking loudly. The oil was icky looking with gunk(spelling) kind of brown. This afternoon after sitting all day the oil is black.

I used another website that said for a knocking noise, it could be the knock sensor.

What is the knock sensor? What is it for? I have never heard of one before.

Anyone have any suggestions as to what could be causing the knocking sound or what damage this water could have caused the engine?

Any and all help/advice/suggestions are greatly appreciated.