What term describes an unpleasant and loud sound?

The term that accurately describes an unpleasant and loud sound is "noise." In audio and acoustics, noise is characterized by its lack of harmony or order and is often considered undesirable sound. It can be random and chaotic, leading to discomfort or distraction.

In contrast, the other terms have specific meanings that differ from this definition. "Interference" typically refers to the disruption of a signal, often in the context of electronics or communication, rather than sound qualities themselves. "Melody" is associated with musical composition and harmony, comprising a series of musical notes in a structured sequence that is usually pleasant to hear. "Echo" involves the reflection of sound waves off surfaces, leading to a delayed sound that can be distinct and not inherently unpleasant. Thus, among these choices, noise is the most fitting description for an unpleasant and loud sound.