96 965: clunk/rattle under L rear seat, hitting bottom of car?

[crashtest]

I have a very annoying clunk coming from under the rear drivers side of my 96 960 wagon. I happens more on small bumps or rocking motions, and sounds like something hitting the bottom of the car. A larger dip in the road gives a nice loud thunk that you can hear and feel.

I crawled under the car, in the snow, for two nights now looking for something loose. I secured the fuel filter and banded down a loose heat shield over the driveshaft. No change. I also looked through the fuel pump access panel. Driving the car with the panel open did not increase the noise - I thought it might be something on top of the tank.

There's a bushing right under the rear door - but it looks solid and I can't coerce any movement.

Anyone have ideas on this noise? It's very odd - it went away for a few days, and is now back and louder than ever. It's not like normal suspension noise as it's silent when entering driveways and even over some big bumps. It is driving me crazy.