Electric brake clatter

I was adjusting the rear brakes on a newly-acquired trailer over the weekend because they weren't doing anything.

As I started to adjust the brakes and tighten them down, I could hear a distinct clackety-clack noise once per revolution coming from the drums.

Sooner or later I'll pull them apart and see what's going on. Probably just put whole new brake assemblies on the rear axle.

Unfortunately I did not have the time, ambition, or parts to investigate further, but I'm curious as to what might be making the noise. Any ideas?