'Um' is a filler. I did it for quite a long time. It started as an easy way to lead into difficult words, and sounded like filling for thinking time, but the problematic part of the brain caught on and I ended up saying "Um um um um um um um..." I had to consciously stop it when I got very annoyed by it. The handy trick turned into another speech impediment. I knew someone a long time ago who used 'actually' as a filler, which wouldn't be too bad if it was the odd one, but he would say it 5 or 6 times in one sentence. Loads of people use fillers in speech these days, not just stutterers. You'll hear 'like', 'you know', etc. all the time. The problem for stutterers (or for me at least) is when the filler becomes part of speech, part of the sentence construction, it is as susceptible to the stutter as any other word.