Rattle is a vocal effect produced in the vocal tract. According to Complete Vocal Institute, this effect can be formed by many different structures:
- vibrations at the arytenoid cartilages
- uvula (palatine uvula)
- back of the tongue
- the soft palate
- mucosa / saliva vibrating over the arytenoid cartilages
Rattle is often used simultaneously with other vocal effects, such as distortion, which adds to its intensity, although it can also be used alone.
References: Complete Vocal Institute