I had a problem a year ago with my Outback developing a strange noise, not a rattle, but a low frequency buzz. It would come and go with different road speeds (windspeeds). I took it to the dealer, and the senior tech thought he knew what it was and we took it out for a spin. Of course it didn't make the noise. He asked me if I had had the windshield replaced and I told him that it was replaced in the early Spring. He said the noise sometimes happens with the replacement rubber seal - it vibrates. Instead of chucking on the spot for a new OEM seal, I decided to experiment and took a short piece of electrical tape and moved it to various places along the seal and bodywork. I finally found the spot where the noise stopped. Then I lifted the edge of the seal in that area and ran a thin bead of clear RTV to "glue" it down. No more noise. Apparently the noise began when the rubber seal warmed up in summer weather and became more flexible. So, curing the "rattle" may be something that simple.