What type of noise might indicate a problem during the coastdown of an engine?

During the coastdown of an engine, a rubbing sound is significant as it typically indicates an issue with the engine’s internal components or accessories. This type of noise suggests that there could be physical contact between moving parts that should not be touching, which may lead to wear or potential failure.

Rubbing sounds indicate that there may be a misalignment or malfunction in parts such as bearings, gears, or other rotating elements within the engine. Detecting this noise during the coastdown phase is crucial because it often allows for early identification of issues before they can escalate into more serious problems requiring extensive repairs.

The other types of noises mentioned, like rumbling, whistling, or static noise, do not specifically indicate internal contact or serious malfunction during coastdown. They might arise from normal operating conditions or minor issues but do not have the same immediate implications on engine health as a rubbing sound does.