My wife's 2005 Expedition makes a sound at startup that at first I thought was extreme high RPM's for a second or two, but it doesn't seem the RPM's are jumping. It sounds not quite like a grinding sound but a loud growl, and lasts for only a second right after it fires. It only does it every couple of weeks or so, really intermittent. Any ideas?

 

Could be many things, but I'd start by checking the radiator fan clutch.