2004 Lincoln Navigator Noise on startup

Noises problem
2004 Lincoln Navigator V8 Four Wheel Drive Automatic

I have a 2004 Lincoln Navigator. Whenever I turn it on it makes a loud rattling sound for about three seconds and disappears. Sometimes it ticks but as it warms up it fades. I think the ticking may be a different noise entirely. But that loud rattling noise sounds bad. Sometimes I start the truck and I get no rattle but for the most part if I let it sit for more than 30 minutes it will rattle on startup. I took it to the dealership and paid $80 for a diagnostic check and was told that they did not know what it was. I do not want to take it back in and pay the same so they can tell me the same thing without getting anything resolved. Can anyone help?