My PS3 does the same thing... it's sitting in the middle of an AV Rack with other gear (open back/front, glass shelves), and after 10 mins, the fans start to get louder and louder.. not quite 360 playing a game loud of course, but loud enough to notice it isn't 'silent'..
It's not surprising, it's well documented that the PS3 kicks out a lot of heat, and if you don't have very good airflow around it, the fans will kick in to get rid of the heat build up.. I've had 3 PS3's under my TV, all behave the same, after 10 mins, the fans wind up to a noticeable level.. I think mine is a little bit louder then some, but I can't see how I could take this back and claim it's a faulyt?