Originally Posted by woodcraft

Something inside the rim only moves around at barely moving speeds, & valve stem rattle is usually also somewhat speed- dependent.

Deeper carbon wheels amplify sounds, so the source can be small.

Glue on the tread, poorly bonded section of tire, base tape thread hitting the brake shoe, debris embedded in the tread, loose spoke, spoke cross clicking, even tire sprue hitting a tight- clearance fork (unlikely w/ tubular tire).