Whenever I drive around, for the past few weeks now, I've been hearing a wierd noise come from (what sounds to be) behind the passenger seat.  It almost sounds like someone put a ball back there and when I slow down or accelerate it rolls around and hits things (I know this clearly isn't the case, but it gives you an idea of what I am hearing.)
Anyone have any ideas?  It kind of worries me.  Car drives fine, but you know how these things go.  Its a clunking noise one minute and a dead car the second!   
