All Sound is vibration. Anything we hear is the result of vibrations in the source affecting the atmosphere around, creating vibrations which further impinge on our eardrums, creating vibrations that we pick up on. The plastic is more rigid, brittle and so displaces more air molecules randomly with more vigour than does a soft texture like a fabric. Change the ambience say by putting both underwater and both would sound significantly damper because even lesser vibrations. Another good example would be a piece of rope moved around vs whipped at speed
This doesn’t explain it like you’re five, but there was some attention to this question in the late 90’s –
[https://acoustics.org/pressroom/httpdocs/139th/kramer.htm](https://acoustics.org/pressroom/httpdocs/139th/kramer.htm) [https://journals.aps.org/pre/abstract/10.1103/PhysRevE.53.1465](https://journals.aps.org/pre/abstract/10.1103/PhysRevE.53.1465) [https://ui.adsabs.harvard.edu/abs/1996APS..MAR.O2305K/abstract](https://ui.adsabs.harvard.edu/abs/1996APS..MAR.O2305K/abstract)
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