engine making ratling noise

I have a 1999 Nissan Altima with 154K, this morning when I started to drive, io noticed a ratling noise coming from the engine. After about 5 city blocks, I pulled over to check my oil level and foound out that I was extremley low on oil. I added two quarts to start but the ratling noise was still there. Any ideas as to what it is, the amount of damage to the Altima, do I have to junk the car?
Any help would be apprecfiated.