There's not just one kind. "Wh- wh- wh- what did you say?" There's been surveys even here in this sub, and the majority responses were that *blocking* is the most common. In my own case, it would be something like "What.... ..... ..... did you say?" or "..... ..... ..... Can you say that again?" or "The... uh... uh... uh... the best way to answer that is to talk to each person individually." Some people have filler sounds, some just stop making sounds, could be at the beginning, middle, or end of sentences. You get the idea, it's a wide, wide spectrum and everyone's is different.