Service Engine Soon

[CrazyDemon]

I was coming home from a friend's house, and I noticed a somewhat odd noise coming from the engine. Barely audiable, i wouldnt even think much of it normally, like a belt was rubbing up against a cover or something.

About 5 minutes later I noticed that the car didn't want to accelerate and it's like the power has been taken out of it. However, when put into Neutral, the engine revs up fine and didn't seem to be causing this. Another few minutes later, the Service Engine Soon light came on, and I came limping home.

After doing some reading, I went back to investigate some more, and the engine seemed fine and the light wasn't on anymore, so I took it for a short drive. The performance definitely seemed better, but something is still off. Maybe it doesn't have as much power or didn't shift as nicely, or didn't have as much torque.

Coming back home, i noticed a knocking sound coming from the heads, like a rocker was smashing up against a valve, or something similar.

Does anyone know what could be causing the problem before I rip into it, or take it to get inspected?
Thanks.