Yesterday I was driving, literally just got my inspection done, and i noticed the check engine light come on.

Did not have the time or items necessary to get the codes, but i just drove to work, parked, and when i came back to it after work it was gone.

Went on the highway, and once i started really getting the RPMs up there, maybe trying to go 60-70 MPH, the light came back on and the car started being really sluggish when i would give it gas.

I was hoping the code would stay till i got home, and it did, but apparently once i shut the car off it clears itself.

I tried getting teh code to come back by driving around my house a few times and no matter how hard i tried, it didnt come back.

Now, however I noticed when I put the car in neutral and rev the engine up, maybe 4-6k RPMs, I will hear a click/knock when i let off the gas.

Does not happen when the RPMs are lower than that as far as I can tell.

Any ideas?

TL;DR
Car is throwing a code, could not get code at the time, when this happens, the car will get really sluggish and unresponsive at times when i give it some gas.
Also, there is a click/knock when I rev the engine high enough in neutral.