The 302, like all Sound Devices mixers, including the 442 and MixPre, can exhibit a static, or crackling sound when gain controls are manipulated when the unit is first powered. This noise goes away as the power supply and filter capacitors become fully charged.
This is normal behavior and is a result of the high-efficiency power supply design which maximizes battery runtime. After the unit is powered up for a few seconds and the pots are rotated a couple of times fully up and down, the coupling capacitors in the unit are then fully stabilized. Before the pots are rotated and the power supply has stabilized, there is a low level of current running through the potentiometers until these capacitors stabilize. Because of this, there can be some initial noise. This design is common to all Sound Devices products to a certain degree, and to many other manufacturers (again, to one degree or another).