I've owned my V-10 Excursion for 10 years now, and the other day I started it up and it started making a sound like a leaf stuck in the blower motor. That was exactly what it sounded like, only it didn't change with different fan speeds, and when I turned the fan off it was still there. It was possibly more of a rattle sound, but very much like a card stuck in the bike spokes sort of noise. It is intermittent. Will sometimes be barely audible, other times it is just kind of annoying, but never really 'loud'. Sometimes it completely does away. There is no pattern to where and when it decides to show up - idle, cruising, accelerating . .. just random. With it running in park and my head under the hood, I swear it's coming from the rear/passenger side of the engine, but it is very hard to tell. It resonates like something is loose and rattling. Does not sound like an exhaust leak (though I know I have broken studs). It does go up and down with engine RPM, but only in frequency not really louder or softer. serpentine belt / tensioner / idler / fan clutch all seem normal, at least no visible sign of being the cause. That was my next hope - maybe a loose pulley but the sound doesn't seem to be coming from the front of the engine. If it was something in the valvetrain or bottom end I assume it wouldn't be intermittent, but would be getting progressively worse.