There's a strong chance that the knock is from a cam timing gear that's starting to come apart. It's a fiber gear with a steel hub, and after many years -- it seems related to age more than mileage -- the fiber starts separating and you get a knock at idle.

Closely related is a worn cam thrust plate (the retaining plate). The cam end float is set only by the difference in thickness between the plate and a spacer ring on the nose of the cam. The plate is supposed to be .003" thinner than the spacer, which keeps the plate from being pinched between the front of the first cam bearing journal and the rear of the timing gear... but both those surfaces contact the plate as the cam is pushed forward a backwards by the lifters. Eventually one or both sides of the plate wear away, end float increases to much more than .003", and you can hear the cam knocking forward and back at low speeds.

It's not a huge job to replace the gear and/or the plate -- the head doesn't need to come off, or anything like that.