Mine does the same.It is a knocking sound but not loud. And also only when engine is hot.Can it be the oil being too thin when hot?

I even listened underneath the car to hear if it is not louder indicating bearing problems on the crank or conrods.It is actually softer underneath the car and louder at the cylinder head.In my case cannot be piston slap,bearings in engine or play at the small end of the conrods.We checked it when engine was overhauled 50 to 60 000km ago.Was no slap and bearing caps were replaced with new ones.Cylinder head completely overhauled.
I am just living with it.