Engine is ticking

For the past month I have been trying to figure out why I hear the ticking/knock sound. The knocking comes and goes it is not constant it really goes away when I am driving and comes and goes at idle. We took it to mechanic to have it checked out and they said they don't know what it is without opening up the engine which they didn't do, but that it could be a valve or a rocker or something. I do have a code reading p2097 running to rich post catalytic converter bank 1. And burning oil inside the engine somewhere. Would the knock really be just a rocker or valve? Would it be best to rebuild my engine or just swap for a built one?