3", 4" and 6" butterfly valves with double acting air to air actuators on them.  The valve seat is a dovetail design.  The actuators have no travel stops.  When solenoid is energized to open the valve the valve disc suddenly pops out of the seat.
Air actuators behave like that in response to loads that change with position, because of the energy stored in the air.  If it's causing a problem, please describe the problem.
rubber-seated butterfly valves are >supposed< to be shipped and stored with the disc in a partially-open condition.  If they sat on the shelf closed for months before installation, the liner may have permanently deformed around the edge of the disc.  This aggravates the "pop" you experience.  One step further, when initially installing the valve- partially open the valve before torquing the flange bolts so the liner does not envelop the edge of the disc as it is compressed between the flanges. 
Raising your air pressure or increasing the actuator size both contribute to actuator "Stiffness" and will reduce the effect/magnitude of the "pop".  Unless you are using the valves for modulating service the pop is merely annoying and is not harmful unless it keeps the actuator from opening the valve at all.

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