Beatboxing with your mouth closed can be used to add effects to a show to represent, for example, a door opening and closing. Performers such as Pablo Francisco and Michael Winslow use this technique quite a lot! If using a microphone, you can cup the mic to your throat to pick up the sounds a little bassier.

To make the sound

1 - Quite simply close your mouth and see what sounds you can do!
2 - Sounds like the 808 kick, 808 snare and dry kick are very easy to do, but don't limit yourself - anything from rimshots to clickrolls can be done
3 - If you're having trouble keeping your mouth closed you can cheat by simply using your hand to pinch your lips closed

Peace, love, and you say it best when you say nothing at all

-FT