"Hmm" is a sound that people make when they are unsure about something, hesitant or deep in thought. There are a few synonyms for "hmm" that can help convey this idea in different ways. For example, "well," "let me think," "um," and "er" can all be used instead of "hmm." Each of these words implies a slightly different meaning, so it depends on the context in which it is used. "Um" and "er" are often used when someone is struggling to find the right words, whereas "well" and "let me think" indicate that the speaker is actively considering a problem or question. Overall, using these synonyms can help to add variety and depth to your dialogue.