Visible crew/equipment: Look over Val's right shoulder as Rhonda is supposedly driving upside down with her legs sticking out the back window of the truck cab and you can see a man driving for her. (00:42:40)
Continuity mistake: When they are picking up Burt and Heather, Burt throws a sack to Val. When it cuts Val catches a totally different sack. (01:15:07)
Chosen answer: The machine is a vibrating case cleaner, which is used to polish the brass casings of pistol and rifle ammo. The "dirt" is actually an abrasive polishing compound.
BGraz