Answer: "quietly" versus "lowly."

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Today, we’re going to look at the difference between the often-confused words quietly and lowly.
And what better place to go to learn the difference between two silence-related words than Silent Hill?

Quietly is the adverb counterpart of the adjective quiet, which means ‘still,’ ‘calm, ‘hushed,’ or ‘not making noise.’ The word quietly itself means ‘in a quiet manner’—that is, the action to which it applies is performed in ‘a quiet manner.’

As the noise of something scuffling across the floor outside the office caught his ear, James set the music box back on the table quietly.

Lowly is both an adjective and an adverb, derived from the word low (which refers to the position of a given noun on a wide variety of scales, including physical height, social rank, and noise volume). As an adjective, it means ‘suited to a low position or rank,’ ‘ordinary,’ and ‘commonplace.’ As an adverb, it can mean either ‘modestly’ (that is, ‘in a modest manner’) or ‘low in sound’ (‘in a low tone or volume’).

An example of the adjective:

“How can you, a father of this church, stand there and refute the idea of birthing God?” Claudia asked Vincent. “We are but her lowly servants! It is our duty to see her into this world!”

An example of the first meaning of the adverb:

“Samael…,” said Dahlia, bowing lowly before the glowing deity standing in front of them. “God’s will has been born!"

And an example of the second meaning of the adverb:

“I’m going to go find the key,” Henry said lowly to Eileen, glancing behind them to make sure there were no monsters or ghosts following.

Although the second meaning of the adverb can be used as a synonym for quietly, be careful where and how you use it; because it has more than one meaning, use of it in certain instances (such as in the first example with James) will create confusion over the context of your words.

Sources
Merriam-Webster
Webster’s New World Dictionary, Third College Edition