I have a 98 Lincoln Navigator that runs great. When I start it in the morning there is a ticking noise that is in time with the revolutions of the engine. The noise goes away after 1 minute to a 1 minute and 30 seconds. When I press the gas the noise accelerates with the engine. I is hard to identify becasue it does'nt stay long enough to pin point it. It only does it if the vehicle sits for a long time. When I park in the lot a 8 and return a 6 it does it. Any hints as to what to look for?