Back wheel clunking noise.?

I have a "96" mercury Villager, and replaced most of the braking system, lines, ABS, Master cylinder. Last few weeks, I started to hear a clunking noise driving down the street. The faster I drive, the faster the clunking. I press the brakes, it goes away, and when I let up it starts up again, but the brakes work fine. Could it be a bearing starting to go out? Could it be bad brake hardware? I replaced all springs and so on including shoes, and cylinders. What could it be?