How do I remove it? I want to open it to inspect my heater fan, I think there may be leaves inside or something, cuz there is a lot of heater fan noise.

 

http://www.innerbean.com/housecor/cabin_filter.html

there should be five screws. two that hold the lock clip. two more in the top corners and one in the bottom where the plastic door molding meets the dash. i also found it easier to do this if you remove the plastic trim piece that runs parallel to the door sill. let me know if you need me to explain further.

 

here is a write up from courtesy nissan that i used. this one gives the location of all the screws and clips. either one of these write-ups should make it easy.
you may find some debris but what you are most likely experiencing is the blower motor ticking. nissan released a tsb on this subject and their cure was a cover that fit around the blower itself. here is a LINK to the tsb on studman's site. it has a part number for the cover if you decide you want to give it a shot.

 

i had a leaf once that got stuck inside my vents and everytime i turned on my fan it would sound like the fan blades were catching on something for a few seconds and then the blower would make a whizzing sound which got worse as you turned up the fan speed.

 

make sure you dissconnet the battery for this one.. otherwise youll get the Airbag light to flash for ya..

 

Or just don't unplug the airbag harness. Just release the 2 clips that hold the plug to the box like it says in the Courtesy Nissan writeup.