2002 Hyundai Elantra rhythmic knocking in rear wheel

A better suspect would be a warped or cracked brake rotor or drum. Start with a basic brake system inspection.

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.

Just to add to what the above technician mentioned, they're are wireless chassis ears out there but typically they're all wired so be sure not to run the wiring where anything moves. Like he also said, glance at the brakes first and then use the chassis ears if nothing is found.