On the passenger side in my 2001 Tacoma Prerunner x-cab. If I press on the lower-left corner of the airbag panel it stops, and I can also get it to stop by pulling on the metal rim that runs below the dash -- also below the airbag area. I have been up behind the dash a couple of times for serious examinations, and I can't find a thing. I don't want to mess with the airbag, though, so I can't just start taking things out until I find what's rattling. I've about decided I'll have to take it in to the dealer and prepare to pay through the nose.