2003 Hyundai Accent Ping Sound, suggestions?

My daughter bought a used Hyundai Accent 2003 during the summer. There was a lot of sludge in the oil and we changed it often until it looked clean. Has been running great and excellent gas mileage (37). She has been getting an P1401 engine code from time to time, we changed the gas cap which sounds like the most common problem. But the code still shows up after a few weeks. Then last week several things happen. The weather turned really cold, her gas mileage dropped to 30mpg, and she got an P0325 error along with a ping sound in the engine. Any ideas/suggestions? Here is a link to the video and sound I'm talking about.

https://www.youtube.com/watch?v=zGK_-6k8XQ0&feature=youtu.be

Any ideas/suggestions?

that code is a knock sensor malfunction, could be the wiring or the sensor itself. ecu uses that to adjust the timing so if its not woking would explain a pinging sound. the noise sounds more like a mechanical issue like a stuck valve or broken valve spring unless its louder under the car then its a bearing issue. valve cover will need to come off to see if you can find a broken part underneath it

We took off the valve cover last night, nothing appears broken. From the top, it looks like it is all working like it should. I expected it to be full of sludge and was surprised it wasn't. So maybe it is a bearing, which isn't worth fixing. We'll prob do an oil change tonight, an STP or treatment of some sort, and drive it till it dies. Hopefully it lasts a while. Unless anyone has any other ideas. Maybe I'll check the compression, although I'm not sure that will tell me anything at this point. I don't hear the knocking at all when it's just turning over and not firing (all plugs wires removed). Not sure if that means anything or not....could that mean it's not a bearing?