A clicking engine noise on a cold start( i.e in the morning or over 6 hours of switch off) that fades after the car has warmed up could be either a piston slap, valve train noise or some other problem.

Using a mechanic's stethoscope to locate the noise would pinpoint the source. Clicking or a sharp tapping from the valve cover on top of the engine would point to a valve train noise. A dull tapping or slapping sound from the side of the engine block would point more toward piston slap. Piston slap occurs on a cold engine when clearance between the piston's lower skirt and cylinder wall allow the piston to rock or cock slightly on the downward power stroke and slap against the cylinder wall, generating that slightly duller tapping sound.

Another possible cause is called CCDI, or combustion chamber deposit interference. This cold start noise is caused by a carbon buildup on the crown of the piston and roof of the combustion chamber, causing physical contact and a relatively sharp click at the very top of the piston travel.

Both noises tend to abate as the engine warms up. With piston slap, the aluminum piston skirt expands a bit as it heats up, eliminating the excess skirt-to-wall clearance. With CCDI, the clearance between the piston crown and chamber roof grows as the engine heats up, eliminating the contact. Decarbonizing the combustion chambers with a professional induction cleaning solution would reduce or eliminate CCDI.

These types of noises are not catastrophic and typically don't mean a big engine problem is imminent. I'd suggest just allowing a bit more warm-up before heading out on the road.