Posted this in my thread about my new 360, but I noticed a difference in how loud my new 360 is compared to my old 360. Old one, didn't run full blast other than certain spots in a game, new one, pretty much fans are going full out even if you're not playing game.
I think, the difference is, MS decided to turn the fans all the way up, so that over heat issues wouldn't be an issue, hence more noise.
New 360's manufactor date is November of 2006, old one was November/December of 05.