Strange Noise - Need Help to ID

[Pilotdave]

Recently we have been hearing a strange noise while driving in our '31 Roadster. Here are the clues:

1. Sounds like a piece of sheet metal dragging/bouncing on pavement or rotating and hitting something.
2. Seems linked to car moving - haven't heard it while car is stopped with engine running.
3. Appears to be speed (car or engine) related.
4. Intermittent
5. Sounds as though it's coming from front of car.
6. There are no pieces of metal (loose or otherwise) hanging under the car that could hit the road. Muffler is firmly in place.
7. All brake rods are properly attached and equipped with anti-rattler springs.
8. Fan is two-blade aluminum and doesn't hit anything when rotated.
9. Nothing lurking inside hubcaps.
10. Starter motor and alternator are firmly in place.
11. Throw-out bearing has been greased - along with all other lube points.

I haven't yet pulled the wheel hubs to look for anything flaky inside...mostly as this noise is louder than I'd expect from, say, a broken spring inside there. But this maybe sloppy thinking!

Would appreciate any suggestions!

Thanks much.
Dave