This can be a nervous system response. Some people hum to 'wake' their system up and keep focused and some hum to calm themselves. You see this with kids that have sensory issues, developmental delays, autism, adhd, etc. Making noises such as humming is on the evaluation. good luck. If you have more detail to add, perhaps there might be some other thoughts. goodluck
Some folks hummm when they feel uncomfortable , I noticed it in a family member once, that if there was any argument he started humming .