First try to tighten all the bolts attaching the seat rails to the car body. It may be just that.
If not, disconnect the seat airbag/sensors, unscrew the rail bolts, unscrew the safety belt screw, remove the seat. Then remove the rails from the seat.
Then what I did - (*and this may be uneccessary, see below) - on each of the two rails, I bent a piece of metal at one side (a bent metal preventing the two parts of the rail to separate while in car), and separated the rails (each in two pieces). A white plastic thingy with balls/bearings came out of the rail, and it took me a while to figure out how to put it back (hint: it moves inside the rail freely).
Now, with the rails separated, I put each into a vice and squeezed them to be tighter, but still possible to put back together.
Don't forget to re-bend the stopper piece of metal once you assemble the rails.
Images of the vice pressing attached, couldn't find more images sorry.
*I'm saying this may be unecessary, becuse maybe you can press the rail without separating it into two pieces. I haven't tried that, may work, doesn't have to. May also damage the bearings inside, not sure.
**Aside from this, while looking for the source of the rattle, I pretty much deconstructed the whole seat as far as I could go so that it was still reversible, tightened all the bolts properly when putting it back together, but I think what did it was either the rail pressing, -or- the rail-to-car-frame bolt tightening.
***Pro tip: don't drive fat passengers. (just joking)