Often you hear people go "um" and "ah" (or alternatively "erm") while they are thinking what to say.
I have come across "um and ah" or "umming and ahing" but this sounds rather informal and colloquial.
I was hoping there is a more formal, single-word verb that describes this action.
I think the right word depends on what is going on for the speaker:
If they are naturally just not a fluent speaker, you'd probably say they are stammering, stuttering or faltering.
If they are nervous, you might say they are hesitating (or, again, faltering).
If they are being evasive, struggling to find words to avoid saying the wrong thing, you might say they are hemming and hawing, umming and ahhing or hesitating.
It's hard to neutrally describe a pattern of speech without imputing some kind of thought process.
My first thought is falter, applying the sense of "behave uncertainly" to speech:
"They ran in here and then... um..." The witness faltered in her testimony.
Merriam-Webster indicates the transitive form for this ("faltered her testimony"), but one could generally say that speech filled with these utterances is faltering or that the speaker is faltering. Note that this doesn't assume the speaker's intent -- thinking, procrastinating, waiting for a distraction to end the interaction, etc.
However, on second thought the right word may depend on the purpose or effect of the utterances. They fall into the category of speech disfluencies, which serve a large number of purposes (differing between languages and cultures which particular sounds are supposed to mean what): filling silence, indicating that the speaker is thinking, indicating that attention is being paid, and so on. In North America it would be perfectly reasonable for someone to say "ah" with purpose and confidence to express acknowledgement, or "um" to break into a conversation. While faltering describes the instances in which these utterances are used to indicate the speaker is thinking, it probably does not apply to every instance of the example sounds you gave.
Those specific types of words are called speech disfluencies, but that is not a verb used to describe what one is doing when they use those words.
This is a bit of a nebulous question since speech disfluencies are used in so many common situations that among some populations they can represent up to 20% of the total words used in everyday conversation.
Verbs used to describe what you are doing when using a disfluency could include :
Johns Hopkins Medicine's site says that saying words like "uh" often is called "cluttering".
If you do this, you "clutter" when you speak.
(hopkinsmedicine.org)
Scottish English has the perfect word for this:
haver: To hem and haw
You can hear it in this famous Proclaimers song.
Unfortunately you will get funny looks if you use it, unless you speak with a Scottish accent. See here for an example.
These words are fillers, pure and simple- the speech equivalent of wheat and fiber inserted into hotdogs because they are cheap materials. These words are similarly cheap as they require zero mental expenditure, but are aimed to give the appearance that the speaker has, like, you know, umm, totally something to say. @Nosajimiki has suggested fillering, which would seem the natural transformation of the noun to verb, but the word doesn't feel natural. I suggest padding which is a common verb connoting the addition of fillers, even in a linguistic context:
From Merriam-Webster unabridged: pad
2
a (1) : to expand or lengthen (as a book, magazine article, speech) by the insertion of additional material that is usually essentially superfluous and often extraneous to the point of being irrelevant and that is usually used merely to artificially bring the thing so expanded up to some desired size or length or that is used for some other usually equivocal purpose (as to add impressiveness, suggest intellectual depth, mask an otherwise distasteful theme) : inflate — often used with out
An example sentence by me: "The other answers are thoroughly padded with worthless fillers."
Granted, padding in writing or speech is not restricted to cheap fillers, as it is one of the oldest tricks employed by students doing homework at the 11th hour, discovering 17 ways to say the same thing in as many sentences. But it fits the bill.
The formal linguistic term for this sort of non-content filler is hesitation noises. (Wikipedia has a cool list of hesitation noises in different languages/generations.) So, for a verb, you might just use hesitate.
(I was going to also reference hemming and hawing, but that's hesitation or waffling words, not semantics-free filler noises.)
I would say that the person is possibly "prevaricating". However, it's important to note that this implies some evasiveness behind the "um"s and "er"s.
For a non-evasive verb, I would probably go with "umming and ahhing", as it's clearly understandable. There are technical terms for it - but they don't really describe it very well!
I'm not sure if there's a specfic verb as such.
You could perhaps use 'ponder'.
If you asked someone a question and they answered with an 'um', you might say one was pondering for a moment.
Anomia, a symptom of aphasia:
Anomic aphasia (also known as dysnomia, nominal aphasia, and amnesic aphasia) is a mild, fluent type of aphasia where individuals have word retrieval failures and cannot express the words they want to say (particularly nouns and verbs). Anomia is a deficit of expressive language. [...] Individuals with aphasia who display anomia can often describe an object in detail and maybe even use hand gestures to demonstrate how the object is used, but cannot find the appropriate word to name the object.
Further reading on expressive aphasia loops back to "disfluency", as mentioned in Tranquilled's answer.
The most common term I've heard for these is verbal pauses.
Also known as verbal fillers or filler words.
I think it definitely depends on the context. If it is a country's leader giving an address, and they are are using some ums here and there, I might say they are thinking, or calculating, possibly processing, making sure they choose the right words to convey the message. Similar to the hourglass, or spinning icon that we get on our computers when clicking on some links, indicating that some processing is being done in the background. This comes to mind, because when Obama was in office he used allot of ums in his speeches.
Another context could be a student giving a speech in front of her classmates. It is likely that the student is not accustomed to addressing a group of people, and is nervous. In this case the ums, likes, and uhs could be described as fumbling, faltering, perhaps even distressing. I'm speaking from experience on this context. I have to battle with myself omit the so called filler words anytime that I have to address a group of people who are more or less strangers.
I had always believed until now the right term was prevaricate ('to speak or act in an evasive way').
But the (multiword) phrase "ummed and ahhed" is accepted.
I don't think the answers referencing dysfluencies and speech pathologies are relevant since that's not the typical reason. It's usually because the speaker is embarrassed, flustered or caught doing something they can't explain.
