My first thought on playing your MP3 is a piece of plastic being
pushed/pulled/twisted into contact with the ends of the fan blades - How
are the motor mounts?

What's the condition of the fan shroud? Any evidence of the fan making
contact with it?

Don't consider either one of these "gospel", though... Weird noises can
come from really weird sources. Case in point:
A buddy of mine had a similarly tough-to-track noise in his Ford
Courrier that started after we went out on rougher roads than we should
have been. Basically buried the truck in pea-gravel. Once we got it out
and started for home, there was a sound under the passenger floorboard
that was nasty enough that we BOTH thought that something had broken in
the front end, and stopped to crawl underneath and find out if it was
even safe to drive it home. We found absolutely *NOTHING* that could
account for what we were hearing (and FEELING - it felt like something
smacking into the floorboard - HARD) It was bad enough that anytime it
happened, I'd jump and worry that something had let go, and was about to
come through the floorboards. But the little beast handled and otherwise
behaved absolutely perfectly in every way, other than making that noise
at seemingly random times. He only ever managed to cause it "on purpose"
one time, but was never able to make it happen "on command" after that,
and it never happened below 25 MPH. No amount of shaking, bouncing, or
otherwise messing with it when the truck was standing still could make
it happen.

Several months afterwards, he was up underneath doing something - I
think he was putting in new brake pads - and found an oblong stone about
the diameter of a dime, and twice as long trapped inside a coil spring
between the frame rail and the floorboard. Removing the stone cured the
noise permanently. Near as we were ever able to figure out, the noise
resulted from a bump that mashed the spring far enough for the stone to
make contact with both the frame and the floorboard - but ONLY when the
stone had managed to jiggle into a "standing up" position inside the
spring.

Strange guy that he was (As if *I* have room to talk about strange - but
that's another story :) ) he had the stone drilled to take a ring, and
used it as a keychain :)

Lactose intolerance?  My first thought was a loose heat shield, but it
doesn't sound like that.  Still.....

If it's that rattley sound you're talking about, that sounds kind of
like the bad catalytic converter on my '93, which started making that
noise one day when I floored it and took it to the ignition cutoff in
first and then second gear.  Inside the metal casing is some kind of
ceramic, which age and impact can fracture into gravel.

yrs jp