Need help, unusual knocking problem.
 

So I'm driving home and my temperature gauge starts reading hot, like almost all the way on the red, then it goes all the way right (overheating) and I slow down, as I come to a stop the temp gauge swings all the way back cold and I start to hear a tapping (knocking)
I Turn the car off and open the valve on the radiator cap, I then go to fill it up with water and its already full??? So I start the car, same thing, knocking and I get the car towed home because I dont want to continue to break whatever is broken.
I turned the car on this morning and still the knocking, Any ideas on what it is, or what I will need to do to fix it, or am I already screwed.

Thanks

if its an actual engine knock,you are going to enjoy installing a new engine.

Doesn't sound good, what level is your oil at?

when i checked the oil, it looked fine, i added a quart just to see if the tapping (knocking) would go away, but it didnt

get us a video. Might be lucky and it be a loose torque convertor bolts.

Is it a tapping or a soft or hard knock. Is it all the time or just certain RPMs? Was the engine HOT when the temp gauge went HOT. It sounds like it wasn't boiling. My first thought was a countershaft knock. I guess because I drove my 97 Blazer almost 100,000 miles with a countershaft knock.