My truck as started making this awful sound. like a chatter,vibration, rattle type noise.
It happens at 800 rpms.
Not every time without fail, but more often than not.
Sounds like it is coming from underneath the truck. I've not been able to really narrow it down.
It almost sounds like a heat shield rattling against something.
Below 800 rpms, nothing, above 800 rpms, nothing.
Doesn't matter if it is in gear or not. Moving or not.

Sometimes it will do it within a few blocks of my house. Other times, it won't make a noise all day, then out of no where, start up again.

It doesn't effect the way the truck runs.
Doesn't feel like the transmission is dragging or not shifting or anything like.
Does it in park, neutral, in gear, sitting still, moving, A/C on, A/C off,

Other than happening at 800 rpms, the only other symptom I can find is that there is a hum/whine that comes from the intake tube when the throttle plate is closed. It isn't really loud. If you weren't listening for it, you probably wouldn't notice it. I almost think that I only notice it because I was paying so close attention because of that rattling noise. The bit that I've been able to research about the hum/whining sound, people have said that it is normal. I don't know, never owned a ford before.

The rattle is engine speed based. You can hear the ratting slow as the rpms fall below 800 and it speeds up as the rpms rise above 800. but by the time you get +/- 100rpm, you don't hear it anymore.

I got under the truck and didn't find any loose heat shields. Nothing appeared bent or anything like that.

I honestly can't think of anything that would be happening at 800 rpms that would cause that.

any Suggestions?

truck is a 2003 F150 with the 4.6 v8 in it. automatic tranny. just over 120K on the clock.
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Videos of the noises.

Link to video of the rattle. chatter noise.
http://www.youtube.com/watch?v=7O4qih5nqP8

and the whine/hum intake noise
http://www.youtube.com/watch?v=ADiFFcBkhIA