If the evap system is working properly and you open while it has been running I don't believe you will ever hear that sound. If it has been sitting in the sun parked and off you might but I,m not sure. The purge solenoid opens and closes very rapidly in succession when called upon by the pcm to do so. Manifold vacumn pulls the vapors out of the tank to be reburned or ran through the charcoal filter, I can't remember which. If the check valve is bad in the cap you may hear a whoosh also.
Move over bear.I'm tired of working on the jeep