At least in part it has to do with sound amplification. It is not an ‘intentional thing’ but certain configurations are more suceptible to generating extraneous sounds.
Harmonics are at times more evident when factors align to assist in sound generation.
Some plastic yoyo molds are thinner/smoother/less complex than others. Most obvious in see through yoyos. Sometimes the molds have a multitude of reinforcement ribs, etc. this formation allows the plastic bodies to be much thinner in spots than others. Often this contrast in ‘thickness’ is the perfect breeding ground for amplification of sound/noise.
On the other end of the complex mold spectrum, consider the Freehand yoyo. Ever take one apart? Pop out the sidecaps and what do you see… An almost continuous thickness of polycarbonate material. The Duncan Freehand halves have ‘no’ thin spots in the molded part; limiting sound generation and actually killing potential noise. < That being said, the Freehand can still generate quite a Racket because the hollow sidecaps are noise generators. Some Freehand with glued in caps are dead silent. Others with pressed in caps or loose caps wail like a Banshee, lol.
Different plastics used in some yoyos, compounded by the various complexity of the molds open up the potential for assorted levels/degrees of sound pollution.
If the plastic yoyo mold exhibits thin and thick transitions of material; the potential for a ‘sound maker’ increases.
And whether it is a plastic body/metal rim or plastic body/plastic rim; the config… of the body combined with the different density of the rings can contribute to the amplified sound equation.
This effect can be further enhanced if the yoyo is kinda vibey. Any inherent imbalance in the yoyo itself on the throw down will direct that said vibration to the thinnest areas of the plastic yoyo body and manifest as noise.
Notice some smooooth yoyos still have a ‘hollow’ feel/and/or sound.
In contrast some vibe generating yoyos are also sound generating yoyos.
Often it is at least a few factors that unintentionally work together to end in a Noisy mess of a yoyo, haha.