This one was mentioned back in the anatomy post, but it comes up often enough in person that I figured it should get its own post.
Question(s): What are those boxes under the seats on the MAX trains? / What’s that buzzing sound?
These boxes sit above either side of the powered wheel trucks and are filled with sand. When the wheels spin or slide on slippery rail, sand is automatically deployed from these to improve traction. This is the source of the buzzing sound you will sometimes hear on the train. Sand is also released to help slow the train in emergency braking situations.