Paradoxish

Alright, I need some advice. I've started to notice a little noise inside the cabin. It's not really a big issue, but it's literally the only one and so it's really getting on my nerves. It sounds like a plastic-on-plastic rubbing kind of creak, but when I hit any kind of bump it turns into a rattling noise. I can hear it with light vibration from bass sometimes as well. It sounds like it's coming from the back of the passenger side front door, near the pillar. It's not very loud right now, but the roads around here are terrible - so I'm afraid it'll get worse (I learned the hard way with my last car to deal with any rattling noises fast).

Anyway, I've checked around the door to see if I can determine what's causing the sound. I've sat in the passenger side while someone else drives. It definetly seems to be coming from near the back of the front passenger door (or maybe even from the front of the rear passenger door). I can't seem to find anything obvious causing the noise. So, any suggestions on things to look for before I have the dealer go over the entire door (something I'd rather avoid since I don't want to be without the car while the spend a day searching for the problem)?