Is there a word for "umming"?

Solution 1:

They are called conversation fillers:

In linguistics, a filler, filled pause, hesitation marker or planner is a sound or word that is spoken in conversation by one participant to signal to others a pause to think without giving the impression of having finished speaking. [...] Fillers fall into the category of formulaic language, and different languages have different characteristic filler sounds.

[...]

Every conversation involves turn-taking, which means that whenever someone wants to speak and hear a pause, they do so. Pauses are commonly used to indicate that someone’s turn has ended, which can create confusion when someone hasn’t finished a thought but has paused to form a thought; in order to prevent this confusion, they will use a filler word such as um, er, or uh. The use of a filler word indicates that the other person should continue listening instead of speaking.

Filler words generally contain little to no lexical content, but instead provide clues to the listener about how they should interpret what the speaker has said. While there are many different reasons for using filler words, sociolinguists have identified six main reasons for doing so: pausing to give time for the speaker to gather their thoughts, speaking more indirectly in order to encourage politeness, approaching delicate topics gently, emphasizing ideas, providing clues to emotions or behaviors, and communicating uncertainty. The actual words that people use may change (such as the increasing use of like), but the meaning and reason why people use them doesn’t change.

Solution 2:

The word for sounds made that are not communicative, such as "er", "uh", "um", etc, and also for interruptions in speech, are generally called "nonfluencies". They indicate the person is thinking of what they will say next, either calling attention to their next statement, or if overused, indicating the speaker is not comfortable with the language they are using, or with speaking in general.

The Toastmasters groups in the U.S. ring a bell (ding) every time a nonfluency is heard from a speaker practicing at a meeting. It's distracting, and meant both to point out the nonfluency and throw the speaker off their game, possibly resulting in more. The idea is that nonfluencies imply unpreparedness and nervousness, which the groups members are there to train out of themselves to improve their public speaking.

However, some studies have shown that speech with no nonfluencies is actually paid less attention to by the listener than speech with a few "natural" nonfluencies. A person who starts an answer to a question with a short "uh" or "er" calls attention to the fact they are about to speak, heightening the listener's attention.