1998 Ford Ranger knock

On the way to work the other day I came to a stop light after getting off the freeway and had a loud nocking sound from under the hood. I pulled over and checked the oil which was a little low. I added 1 1/2qrt. And the noise got softer. After work I drove to a friends house were he said it sounds like a rod knock. Having no other car to get to work I drove it today. Going to work it was the same as the other day, but on the way home I have to go up a steep incline were the noise got alot louder. I cant find a leak so dont know what could have caused this out of the blue like it happened. Will it be cheaper to find a new enginge or can I take it to someone to fix it?