For me, on the passenger side there is the hard plastic cover piece that the seat belt comes out of. This piece extends from the back of the small window to the front where the door closes. At the front most part of it, it hooks around the window frame. At this hook point is where the rubbing/squeaking occurs (at least on mine).

I placed a couple fingers in the seat belt slot and tugged/wiggled the hard plastic cover to create a slight (very slight) gap. And I haven't heard the sound since.