There are quite a few possibilities for your noise. Some questions:
1. At approximately what RPM does the noise appear?
2. Does engine temperature make any difference?
3. Does throttle position make any difference? Light throttle; heavy throttle?
The possibilites are lifter/tappet noise, loose piston, loose connecting rod, detonation.
Detonation will often increase in intensity with a light increase in throttle, and may diminish with heavier throttle. If that happens, disconnect the vacuum advance and see what happens. If there is no more noise, the timing may be too far advanced, or the vacuum advance may be coming on too much. If the noise gets louder with heavier throttle, advanced timing might still be the cause, so check that and retard the initial setting some to see if anything changes.
A loose piston will usually show up as a light tapping sound at light throttle and at moderate engine speed. The sound also doesn't change change much with throttle position.
A connecting rod knock will usually show up at light throttle, at moderate engine speed, and often worsens as you lightly release the throttle. Rod noise usually disappears completely during deceleration. Depending on conditions, rod noise may also diminish or disappear completely with heavy acceleration.
Lifter/tappet/rocker arm noise is normally constant, regardless of throttle position, and sometimes doesn't appear until above a fast idle. But that situation isn't very common. Lifter noise is more likely to appear at low engine speed and to disappear as engine speed increases.
If the noise exists mostly with the engine cold, piston slap (loose piston) is a possibility. But lifter noise is also common with cold engines.
I don't know how much this helps, but it gives you something to look into.
Harvester
