You said its a slow noise, which makes me doubt, but it could be piston slap. with an idle at ~ 600 rpm that's kind of slow if the impact is irregular.
Piston slap is impact of the piston (usually piston skirt area) against the cylinder wall. Its sideways movement of the piston against the cylinder bore, not up and down motion. Every piston driven engine and even most compressors have a certain amount of piston slap; usually when the engine is cold and the pistons haven't heated enough to expand. Excessive piston slap happens when the piston to cylinder liner clearance is too large; usually through wear. Piston slap usually gets quieter as the engine gets hotter.
Of course, really sloppy mismatched pistons and cylinder bores at manufacturing can also be a possibility, but I've always thought sloppy manufacturing of that sort was more a GM/Chevy thing. Their forums are full of guys whining about piston slap.
If your noise stays as the engine warms it could also be rod noise. That tends to get louder as the engine gets hotter. If a rod bearing is damaged you'll also notice your oil has a silvery look to it when you change it.
If you change your own oil, you can also cut open your oil filter and inspect for metal which would potentially confirm a more significant issue.
Hope its nothing, or just a pulley.