Any question answered...

I rode my road bike in today and noticed a rhythmic brushing noise from the front brake, only when using it. It makes a tttthhhhhttt noise, not a clunk, exactly once every wheel rotation. I checked the rim and tire and there's no scratch, seam, etc. Doesn't seem to be anything stuck in the pad - and anyway, it occurs on a wheel rotation, so I'd have thought it was rim/tyre. Any ideas?

Have you checked that the brake is accurately centred to the rim, so that both pads touch at the same time? If one side is contacting first, or with more pressure, it could exacerbate any imbalance in the wheel.