You should use your words better, the word "Knock" is usually used to reffer to The noise you hear when there is detenation.
What your stating sounds like a leaky exaust manifold to me...if it were HLA's the ticking/tapping noise would be there all of the time and would speed up as RPM's increass, but if you hear a ticking noise only when the car is cold it is usualy a leak in the exaust manifold...the reason why the noise goes away after the car warms up is because the crack will expand shut from the heat.
Take you exaust manifold off and send it to a Machine shop to be tested for cracks.. but before you do this, check for broken/backed out studs, or a leaky gasket
Hope this helps.
Methman