2005 mini cooper has loud humming noise from rear when driving above 40 mph. Any suggestions on what this could be

I had a similar issue and it ended up being a broken wheel bearing. I even ended up having two, one was broken on my front passenger side, and one broken on my rear driver side. The part costs about $280.00, but could cost around $400.00 with service repair. The noise of a broken wheel bearing can sound like a humming, and over time it becomes louder. The car can be driven with a broken wheel bearing, but doing so can eventually lead to other wheel/tire maintenance. MINI's endure this commonly due to their low profile to the ground. Here is a forum on further issues experienced, including the bearing, and efforts towards recalls. http://newmini.wordpress.com/2009/02/17/mini-cooper-power-steering-failure/#comment-884