So I can speak some on this.. I am betting that fabric on the ceiling has been replaced(not stock), more than likely due to the glue wearing out and causing the fabric to fall(very common, there are several other threads). To get properly replace the fabric on the ceiling, you need to take out the plastic that lines the trunke(which are just two big plastic pieces). This is needed to be able to drop down the ceiling so it can be taken out of the car so that it can be properly resurfaced. The only problem is that it is like a jigsaw puzzle to get back together when it comes time to put it all back together.

IF you are lucky the plastic retainer clips on the inner side of the plastic panel(the part that you can't see), are still there and not broken. If they are broken you will need to pick up some new ones(pretty cheap if I remember correctly). If they are still there though, you won't know until you take the piece back off, then all that needs to be done is to climb into the trunk(if you are small enough) and start gently pushing the plastic pieces so that they line up properly and the clips pop back into the holes they are supposed to.

Also it is quite common for the plastic that covers the top of the bulk head(right behind the seats) to start cracking due to people putting the seats back too far and causing pressure on the plastic, causing it to crack. I think there is another post about how someone made a fiberglass cover to replace his.

All and all that should help explain most of the plastic creaking.. unfortunately those aren't the only sources. I sometimes hear creaking coming from parts of my dashboard and even the doors at times.. I believe its the type of plastic that they used in certain places on the inside of the car.