Vaporization of the liquid between the lapped faces causing the faces to rapidly open and then close as the leaking vapor cools the faces. A discharge recirculation line is aimed at the carbon seal face. Each pass of an impeller vane puts a pulse into the recirculation line. The pump is cavitating. Remember there are five types of cavitation.
Water hammer is an another possibility.
Running at, or passing through a critical speed will cause vibration problems Pits in the carbon face. Air trapped in the pores of the carbon expands and expels pieces of the carbon when the seal faces get hot. Prior to ejection polished patches will be visible, usually with small cracks visible in the center. If the product solidifies between the faces it will tear out pieces of the carbon at start up. This is a common occurrence with ammonia compressor seals because petroleum oil is mixed with the ammonia and the oil can coke at the elevated face temperature.