After 16,000 miles my rear wheel (Powertap SL+, Velocity Fusion, 32 DT 2.0/1.5mm Revolutions, alloy nipples, 120kgf drive side tension) spontaneously developed an annoying rhythmic clicking noise while under light load that's similar to what happens when the wheel mounted magnet hits a speed sensor that rotated too far inboard.

It's silent coasting, accelerating, and climbing. Just riding along it's not.

It's not the magnet hitting.

The problem goes away when I change wheels.

There was no rain or puddle that could get the spoke crossings dirty between my morning rides yesterday and today.

A cursory inspection shows nothing obviously wrong - it's true enough the brakes aren't rubbing, none of the spokes look like they're pulling out of the rim bed, none are too slack.

What should I suspect?