Q: Engine is knocking loudly and has a weak acceleration.

When we left this morning, the car ran normally. We stopped at the gas station and as we were pulling in we started to hear a slight knocking. As we pulled further in, the knocking became louder. We checked the oil level and noticed it was low. We hadn't kept a close check on it recently as we should have. We filled the oil to the correct level. The car engine continued to knock and when I tried driving it home I noticed a huge problem when trying to accelerate. It moved very slowly at first and finally kicked in and gained a little more speed. The car has no problem starting but continues to run with the loud knocking. The car is a 2005 Ford Five Hundred. I've owned the car for about a year and a half and was told it had a new engine put in just before I got it. The knocking sounds like it's coming from the top of the engine. My car has 157000 miles.
My car has an automatic transmission. *Update: We drained the oil and there were no metal shavings in the oil