My dd5 I do not believe has adhd, but at this age she is doing a lot of asking a question over and over without actually listening for the answer...I have to really get her attention to even answer her (she may be too focused on the actual thing she is thinking about?)
Not sure, but might just be an age thing.
They way you describe it, though, it reminds me a bit of stuttering, and how to handle that: don't freak out/reprimand/raise your voice, just look at them calmly until they are done with what they are asking and then answer them as though nothing is wrong. Seems to downplay their own focus/anxiety over getting out the words/thoughts. If they are not even looking at you, a gentle hand on the arm might be all that is needed to let them know they have your attention.

Ok, I know that could be completely off base, but now that I've been struck with the similarity, I am going to start trying this with my kiddo to see if it helps.