I know from reading other posts from Z owners with the Password: JDM shift knob there tends to be a rattle when the engine is revving. I did two things that cured it:
Next, get a rubber grommet like this one (cost me less than a dollar at a home improvement store):
Place it on the shift lever (at first I tried it under the shift boot, but that didn't work. But here's a shot of how to remove the trim around the shifter anyway, then the grommet on the shift lever:
Remove the little collar inside the shift knob (I think this is the source of most of the rattling):
Reinstall the knob, and you should be rattle free!
__________________
Rain, snow, sleet, hail: 2009 Mazda 3 sGT HB
Everything else: 2009 370z, PW Touring 6MT