Is a pyroclastic flow quiet

Answer 1

That depends on the sense in which you mean silent. Pyroclastic flows are generate by explosive rather than "quiet" eruptions. However, no sound escapes the flows themselves. Their unusual acoustic properties ensure that the sound they produce remains trapped inside.

Answer 2

No, pyroclastic flows are extremely loud and can produce sounds similar to thunder or jet engines as they travel down the volcano at high speeds. They are formed from superheated volcanic gases, ash, and rocks that move swiftly down the volcano's slopes, often reaching speeds of over 100 km/h.