My suggestion to you guys would involve a second person doing the driving while you ( the one that is aware of this rattle/noise) takes a seat in the back seat and do some searching. If you're driving yourself, it's REALLY hard to investigate where it might be coming from, so having a second person doing the driving would probably help you a lot.
My first guess is either a slide tab or a locking plastic retainer. A lot of dealerships make the mistake of using the same ones they just took out.....the reality is; once they are disconnected, they should be replaced with new ones...as any quality body man can attest !
Slide tabs are interlocking little "tongues" that engage into an opposing "slot" these are usually found in plastic panels that interlock into each other. Generally, unless they have been forced OR bent, they cause no issues, but if they were bent out or in, the "play" that occurs between the two will create a weird squeak....Rear passenger plastic panel in my mustang had to be removed when they replaced a seat belt tensioner where the belt comes out the rear panel....the tensioner's seat belt goes through an small opening in the plastic cover...After they removed and reinstalled it, it developed a nasty squeak, when I complained about a NEW squeak that developed since the tensioners replacement, they found that one of the forward facing tongues (4 in total) was twisted right out and was NOT engaging, they ended up ordering a new plastic panel with intact sliders and so sooner it was installed, the squeak disappeared!