I drive on a lot of bumpy roads and dirt roads. Yes my Vibe creaks, especially the drivers seat. It only has 6,000 miles, so I'm going to wait a while longer before I bring it in. My '04 also had a creaky seat and various rattles.
I have 4000 miles! Mine doesnt seem to be coming from the seat though, it sounds like the plastic cargo area... i think, I dont know what else it could be back there.
Take the spare tire cover and the other cover out of the car, and try your drive. If the noise goes away, go to your local home supply store and get some adhesive-backed felt pads to put on contact points between these items and the car. It will take some testing, and fiddleing around, but I was able to silence mine fairly easy.
Current Ride 2015 GMC Terrain SLT
2nd Vibe 2006 Vibe AWD Stealth Monotone "Recon" December 2005 MOTM
Original Vibe: 2003 AWD Abyss Monotone "Darth"
GM/ASE Certified Parts Manager.