In the last playlist update (pushed out on 3rd May 2011) the Noble and Defiant map packs were merged: Halo Forum Update Changelist Post.
This means that the Defiant playlist that appears under the DLC does actually include the Noble maps as well. Unfortunately the Defiant map pack DLC is required to be able to use this playlist. One solution for you could be to purchase the Defiant map pack, however, the Noble map pack is not a requirement for this playlist, which means that you will only get Noble maps on the voting screen when all of the players have the Noble map pack too.
If you don't want to buy the Defiant map pack, then the only other option for you is to use the normal playlists. The Noble maps have been integrated into the normal playlists according to the November 30th 2010 Changlist. This means that you should theoretically still be able to play the Noble maps, however these will only appear on the voting screen when all of the connected players also have the Noble map pack, but as you have probably noticed, the chances of this happening are pretty slim.
This quote is from the Bungie.net Noble Map Pack FAQ:
Many other playlists will contain DLC
maps as well, but since you are not
gated from entry to these playlists by
having DLC, you could match with
people who don’t have the maps. If
that happens, you will not get them as
voting options for that session.
You may be able to improve the chances of you playing one of the Noble maps by using the playlists that require less players. I think that if all connected players do have the Noble DLC, then at least one Noble map will show up in the voting options.