Sounds like piston slap. When your engine is cold the pistons are smaller thus have more clearance in the bore. When u get some heat into the engine they expand @ the skirt of the piston and take up some of the clearance, thus less rattle. Could also be gudgeon pin to piston clearance or conrod bearing clearance. Does it sound uniform thru the engine or do u think its could be localised to 1 cylinder? If u think it is 1 cylinder only, remove the spark plug lead while the engine is running and see if the knock disappears or changes pitch. If it does then it is more likely to be a conrod bearing or gudgeon pin problem rather than piston slap. The vt commodore with the Gen3 was bad for piston slap when cold,a lot of the problem with then was attributed to the short piston skirt.