Emilios,
I had this problem and the cause was a loose valve guide. The symptom was exactly as you describe. The engine would sound nice and quite when cold and as it warmed up it sounded as though a tappet was loose. I adjusted the tappets several times and the same thing happened every time. I would start the engine and it would sound quite until it was warm. Another clue is to look for oil being burnt. If the valve guide is loose oil will get into the comustion chamber. A loose valve guide will not always be noticed when the head is removed because the head will be cold and with a cold head the guide will seem to be OK. However if you warm the head up enough the guide will become loose.
John