Fluttering noise from front end, only when coasting

Our van has recently developed a "fluttering" noise (a little softer than a clatter) that sounds like it is coming from the front driver's side wheel area. Noise appears only while coasting. Most easily heard between about 25 and 45 MPH. If I apply the accelerator, the noise goes away almost instantly. The noise is still audible, though not as loud, in neutral while coasting. The flutter is far faster than the rotation of the wheels (e.G, not a rock in the tire, etc.). The speed of the flutter does vary somewhat with the speed of the vehicle (slower speed = slower "flutter"). The "flutter" is very regular - like a muted version of what you would hear when you put a baseball card in your bike spokes as a kid. Occurs regardless of uphill or downhill; cornering and braking do not change the noise at all. Does not happen at all in park. Pulled front driver's wheel; nothing unusual noted in calipers/drive line/etc. Recent work: full brake job (all four rotors replaced, plus new pads), new front tires, front end alignment. First noticed noise about two weeks after this work.