From popular science:

“Louis Allamandola, director of the Astrophysics and Astrochemistry Lab at NASA's Ames Research Center, told Popular Science that this smell is due to the polycyclic aromatic hydrocarbons floating around in space.

These hydrocarbons can be found in "tobacco smoke, car exhaust, and sometimes in heavily browned foods," here on Earth, according to chemist Kevin Boudreaux from Angelo State University.”

Edit: I just read some more about it and an alternate explanation is that the random elements floating in space are reacting to the oxygen pumped into the airlock. They are oxidizing quickly and therefore smell like burned metal.