A light 'knock' that is continuous, with 173k miles, might be a hydraulic lifter that doesn't hold pressure. A mechanic can determine if that's it, and which side it's on, with a stethoscope. You can do it yourself with a long screwdriver - put the big end in your ear, and the small end to the left cam cover, and then the right one. If that's it, it's not too serious, and isn't too hard to fix.