I have the same issue. I haven't had time to fix it yet, but other people have. This helped me (although there it seems to be on the other side of the car):
Link:
Annoying 05 Wagon Dash Rattle/squeak!Not a very detailed thread, but the two important posts are:
Mine comes from the drivers side a pillar where it meets the dash at the windscreen
I had this exact same noise from the same spot. I got a thin strip of black rubber and pushed it between where the dash meets the a pillar and it stopped. Cant see it either so it was a win win.
EDIT: Pulling the pillar trim off is fairly easy. That gave me access to the area where the above guy put a rubber strip. But I haven't had a chance to do it properly yet. My first quick attempt made no difference - but that was only under the pillar, not between the dash and pillar. Let me know how you go, it seems that a lot of Libertys (both pre and post facelift) have this issue. Would be good to get more detailed info on what exactly needs to be done.