On my '94 Ford Explorer (4x4 with automatic hubs) I have a whirring sound, like a baseball card hitting bicycle spokes, coming from the left front wheel. When I apply the brakes firmly the sound goes away, but it's there if I'm just coasting or lightly use the brakes. I've replaced the pads, and from what I can see the calipers and discs look fine. Maybe the disc needs to be resurfaced? Any ideas?