a mechanical issue would probably make sense. at this stage i'm just trying to keep the facts as straight as possible and not come to any premature conclusions, but i have to admit that something mechanical (like a bearing) gone loose would make a non-dissimilar sound at speed.
peli: my civic has no aux jack; doesn't even have the aux harness behind the centre console
the metal clanging issue would definitely be worth a look into though.
what's strange is i've noticed that while the general tyre roar / road noise has been subdued a bit (thanks to dynamat and ensolite), the sharp shocks/bumps are much louder than i ever remember them being. a loose engine mount/similar would explain the sharp audible signals i get over bumps.
i don't have a good microphone, but i'll charge up the dv camera and hopefully get some useful audio on that.