I have a 1999 SR5 TRD Tacoma, and my passenger seat has the rattles. I only hear it over large bumps. And there's also a funny little noise in the dash above the driver-side air vent. I don't know what it is. About the seat, my buddy that works at a Toyota dealership told me that the noise in the seats is a defect, and that all Tacoma's have this problem. I don't know if I can be of much more help.
Let me know if you find out what is causing the squeak in the dash. Thanks!