Piston slap is the most common cause of an audible engine KNOCK that goes away after a short run time, but these engines don't really have that problem so I would lean more toward lifter noises.
Collapsed/sticky lifters are the most common cause of an audible engine TICK that goes away after a short run time. The cause of sticky lifters is almost always due to lack of maintenance.
When an engine is shut off, some of the cam lobes will stop in the "open valve" position. As the lifter sits with all this valve spring force against it, the inner lifter plunger will bleed the oil off and will collapse into the lifter body. This is normal.
When varnish, sludge, and carbon build in the oiling system from lack of oil changes, it collects between the lifter body and plunger as well as everywhere else. When this build up gets bad enough, it will cause the lifter plunger to stick in its collapsed position for some time after start up. If it gets bad enough, it will make the lifter stick permanently.
I have heard that certain crankcase cleaning chemicals can help to break down and remove the sludge, varnish and carbon and in turn get rid of the sticky lifter condition and the noise it produces. I have also heard that this process of loosening and removing these deposits can cause even more damage.
If the deposits are big enough and plentiful enough, they can clog the oil pump pick-up screen as they are being removed from other engine parts. If the screen plugs it will starve the oil pump and in turn will starve the engine of oil.
Personally, I don't care for any "mechanic in a can" products such as this.