whenever I pluck my low-e string i can hear something shaking, like theres a loose part. it's definitely coming from the neck are, and its not the tuners or the nut. It it possible that my trussrod is loose in its cavity?
The odds of your truss rod being loose enough to rattle are very small. In my experience the most likely source of rattle is the bridge, usually one of the saddles. But don't worry about it; it doesn't cause damage and it doesn't harm your sustain excessively.
It could be that your nut is to small for your gauge of strings. Have you recently gotten a bigger gauge of strings? The string could not be sitting properly in the nut and be making the noise.
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It's incredibly unlikely that it's your truss rod rattling. It's almost in the realm of impossibility. A more likely thing is it's the nut or fretbuzz. I know you said it's not the nut, but it's worth another shot.
Well...it could be the trussrod. But the vital clue would be measuring the action in cubits.

If you've eliminated fret buzz, take the strings off and check each machinehead individually. Check each part - especially any ferrules on the face side, or any screws on the back. Check the tension screws for the butterflies (if applicable).

I've just had to have a good sort out session on my BCR because of exactly the problem you've described!