Well, my first thought would be a tire, but I also have had a noise and sometimes hard to pin point. Mine is actually the belt tensioner, or one of the 2 idler bearings. A few months ago I replace the serpentine belt, and when under power it made no noise but as I slowed down it appeared and when I stopped at idle it would not make the noise. I have driven off and on and now it's much louder and starting to squeal, now it will be easy to determine the exact part to be replaced. There is you radiator fan motor and maybe it could be causing your problem too. Let's us know what you find.
