2002 XK8 "ticking noise when first started"

I am the second owner of a 2002 XK8 Jag 4.0. When first started, the idle is slightly higher for about 20 seconds as expected. When the idle drops, there is a noticable "ticking" noise coming from the front of the engine. The noise will remain until the enging warms up, usually about a minute or so and then gradually goes away. I have taken it to the dealership on three occassions and they can't find the source. The car has 19K miles and no historical problems or accidents. Car runs normal otherwise.