2003 Hyundai Accent Engine Ping

This is my daughters car. The weather got really cold (20ish, cold for us) she got an engine code P0325 (engine knock/ping), and this sound started. She also has seen code P1401 which is often a loose gas cap. Her gas mileage has been good (37) until recently dropped to 30 along with the engine sound and P0325 code. We checked and the oil was insanely low although no light had come on. It had been full two months ago and hadn't seemed to be burning oil prior, although she just bought it in the summer. Any ideas?

[warrenhenke]

Still getting the error, but she's put several thousand more miles on it. We changed the oil every couple hundred miles for a couple times after she had that problem and each time, the sound lessened until it went away.