um and aah
ˌum and ˈaah (about something)(also ˌhum/ˌhem and ˈhaw less frequent) (informal) speak but say nothing important because you need more time to think about a problem, matter, etc: He ummed and aahed for about half an hour and then finally said he would lend me the money. ♢ After a lot of umming and aahing, he finally said yes to the plan.
Um, aah, etc. are sounds that people make when they hesitate or do not know what to say next.