Does it sound like something is moving/rolling around? Has anything been dropped into the floor vent under the seat (lipstick, marble)?
It's possible the seat bolts aren't torqued properly, and the shift in weight is causing the clunk. Have a Service Center check it out. I had a VW Passat with this issue, from the factory. I removed the bolt, applied LocTite, re-torqued the bolt to spec, problem solved.
(FYI: in my experience, if you're close enough, just drive to the SC and ask for advice. They might be able to quickly evaluate your situation and correct it.)