Some people notice this more than others. I notice that kind of thing right away too. You get used to it. Make it as tight as possible. Or even bungee it tight to the seat rails, that way it does not move as much.

The TUBUS "Fly" rack is only 350 grams. You can attach it to the seat stays with P clamps. I think it's a lot lighter than any seatpost rack.

edit post: I stand corrected 325 grams.

http://www.lickbike.com/productpage.asp?PART_NUM_SUB='3590-00'