Undertone (noun)

1. A subtle or subdued feeling, quality, or attitude that is implicit or underlying in a situation or expression.
2. A subdued or underlying sound or vibration, often heard in the background or beneath a more prominent sound or noise.

Origin:

Composed of the prefix "under-" meaning below or less than and the noun "tone" meaning a particular quality of sound or voice, or a particular style or manner of expression.

Examples:

1. There was an undertone of sarcasm in his voice.
2. The movie had an undertone of sadness throughout the story.
3. The song had an undertone of optimism.
4. Many people could sense the undertone of tension in the room.
5. An undertone of anger could be heard in her voice.