This operation is normal. During normal purge operation while the engine is running the ECM will cycle the EVAP VSV On and Off, allowing vacuum from the intake manifold to pull air through the EVAP system. Specific driving conditions, fuel tank level, and atmospheric pressure will determine when and at what cycle the VSV will turn on and off.
Toyota EVAP Purge Operation - When your Toyota Camry's engine has reached predetermined parameters (closed loop, engine temperature above 125f, etc.), stored fuel vapors are purged from the EVAP canister whenever the purge VSV is opened by the Engine Control Module (ECM). The ECM will turn on the purge VSV. The ECM will change the duty ratio cycle of the purge VSV thus controlling purge flow volume. Purge flow volume is determined by intake manifold pressure and the duty ratio cycle of the purge VSV. Air is allowed to enter the EVAP canister to ensure that purge flow is constantly maintained whenever purge vacuum is applied to the EVAP canister.