Which of the following is a sign of machinery "fouling"?

Fouling refers to the accumulation of unwanted materials on machinery components that can negatively impact their performance. Clogged filters or passages serve as a prominent indicator of fouling. When these components become obstructed, they cannot function properly, leading to decreased efficiency and potential damage to the machinery.

In the context of machinery, proper fluid flow is essential for efficient operation. If filters and passages are clogged due to fouling, the machinery cannot draw in the necessary fluid, leading to a variety of operational issues. These include overheating, decreased power output, and potential failures, all of which can result from inadequate lubrication or cooling. Recognizing clogged filters as a sign of fouling allows for timely maintenance actions to restore machinery to optimal conditions.

The other options, such as improved efficiency, reduced noise, and increased power output, would typically be associated with clean, well-maintained machinery. Instead, they are contrary to what one would expect when fouling is present, making them less relevant as indicators of this condition.