Solenoids are a new monovision that appears in the DLC Rise of the Martian Bear . Vlad created these monovisions with the basic purpose of destroying you. Once they are summond they will head straight for you or the nearest player. The basic way to kill them is to keep moving and take them out from a distance. If close enough to you they will spit out several bolts of energy that do only small amounts of damage - but they fire very quickly, making swarms of Solenoids deceptively dangerous. They pose a major threat to Engineering trenches, and in a big enough pack, can seriously threaten even a Soldier or Heavy trench.
They make a distinctive churring when leaving the gateway.