Did you check timing gear backlash? A four cylinder has two valves opening at once and then two valves closing at the same time, so the valve springs will "push" on the cam lobes. As the cam transitions from the opening to the closing phase, it will rock the cam gear back and forth. This changes the gear lash from the front of the teeth to the back of the teeth. If the gears are worn, you will heaar a 'tick' noise; the looser, the louder. It is most noticeable at slow idle Try hand cranking the engine with the spark plugs out. You will hear the gears click every revolution if things are loose. I had a well-worn '29 in high school back in '60-'61 that did this. Thought I was real smart when I figured it out!
