My son is two years old, and I think on the whole his language development is quite average. However, when you ask him a question he often spends a long time going ummmmm ummmmm ummmm.... before answering. I wasn't too bothered by this (I think its quite cute) and it seems to be either when its a question that he genuinely has to think to answer - "what did you do today" otr a question he doesn't want to answer and is playing for time "do you want to wear your blue t-shirt or your red t-shirt" when he doesn't want to get dressed at all.
However, my partner has started telling him not to do it (telling of is probably to strong a word but think gently saying don't do that) when he starts. My first reaction is that our son isn't actually doing anything wrong by ummming, it isn't a swear word and he is maybe too young to expect him to think quietly. But... then I wanted to check if this is normal, and whether it is a behavior we should be correcting or not.... I am sure if an adult "ummmed" for as long it would be annoying but I was thinking it would correct itself in time on its own.