Humming while beatboxing is a very common technique and is particularly useful when it comes to song covers. It's impressive to people who don't know too much about beatboxing, and the technique has been applied to legendary routines like Drop It Like It's Hot.

To do the technique

1 - You need to be humming through your nose - you do this by saying "mmm"
2 - Using your mouth, force air past your lips without using any air from your lungs
3 - As long as you aren't using any air from your lungs you'll be able to hum at the same time

Make sure you use your tongue, jaw, and lips together to keep the sounds smooth. It may take a while before you can get any kind of power to the sounds, but keep practicing.

Peace, love, and big scrummy dinners

-FT