Take it to a parking garage where you can hear the echos. Then you can diagnose it further with the windows down to figure out under what conditions it happens. Is it a repetitive sound that increases and decreases with speed changes? One thing to check is whether you've got a rock or a nail wedged between the treads. It could also be something rubbing, a warped rotor, bad bearing, or loose axle nuts that need to be torqued.
Just yesterday there was an Elemental thread about torquing his nuts.
Just yesterday there was an Elemental thread about torquing his nuts.
