My car is a 2016 Chevrolet Malibu Limited LT. I started to hear a small knocking while I was driving the car so I tried looking around and the only time I could hear the knocking louder was when I took off the oil cap. It hasn't caused an issue with the car yet it has been 2-3 weeks like this but I would like to know what it could be. Lastly, I have also noticed that my RPMs are no longer idling where they used to be. My car used to idle at around 800 RPMS after it was done warming up but now it idles at a little over 500 and when at stoplights it'll idle at around 350 but it doesn't sound like it's going to shut off.
The current codes my car is throwing at me are P151E and P26C8.