The propeller is made of different blades. Each blade causes a pressure diference between their sides in order to create a lift force which moves the ship.
If you take one point of the propeller disc and looks how the pressure changes during the movement of the blades you can see a pressure pulse caused by the blades when they passed across the point. This pulsation spreads througth the water and reaches the hull, causing noises and vibrations.
These type of pulses are very dangerous for the plates of the hull.
