"I liken the sound to a rectangular cookie tin rolling around loose in the trunk"

I had this at low speeds too, found it was the backing plate behind one of my brake assemblies that was just kissing the disc at certain low speeds on just the right road surfaces. It's just a piece of sheet metal that rings like a bell when struck. Solution - just poke a stick through the wheel against the plate and push it back away from the disc a little bit. Cookie tins are now banished back to the kitchen where they belong.

Steve

 

Thanks for the idea, I'll take a look at it.