to drop the headliner, you must remove the a,b, and c pillar plastics. remove the front overhead console lamp assy, remove the sun visors, remove the "oh ****" handles. then there are a total of 6 push in retainers, 4 in the rear and 2 in the front. then you pull down and it will pop off. also dont forget to disconnect the A-pillar electrical connectors. the rattle that you just mentioned sounds more along the lines with the side curtain airbag's. thats what was making noise for me, i took felt padding and insulated the areas the air bad are, i also rapped up two wire looms that run from the front to the back of the head liner, the looms are yellow in color and contain one wire in each. the problem i believe that I had was that the one wire was able to float around and cause a rattle sound in the looms. also my microphone wire separated from the glue that was used to hold it down so that was making some noise.
I resolved all my rattles except for one in the passanger rear, i have it isolated, just need to spend the time and drop down that area and see what i can do.
if you do try and drop the head liner, i advise you get spare clips to and push pins to hold the sucker back up, the originals are in there good.
in all honesty Id actually say let the dealer do it because the head liner is semi flexible, and if you bend it to much you will cause a crease and ruin it.