Interjections is the correct term. You do it because at some point in an effort to get out whatever sound you were stuck on, you consciously/unconsciously threw in an "um" or hum and it helped at the time. It sometimes worked, sometimes didn't, but in our desperation to get past a "stuck" sound we'll try anything and the times it works, it becomes reinforcing. Eventually this develops into a conditioned behavior and you'll try it whether it works or not. It's pretty disruptive to communication, oftentimes more disruptive than actually stuttering. This can make it difficult to unlearn. The fact that you're aware of it is a good thing. I would recommend you do your best to avoid using interjections as they are another learned, unnatural avoidance behavior. You don't have to be on guard every single time you open your mouth, but find a few speaking opportunities each day and work on not using interjections.