This phenomenon is mainly caused by too high or too low oil level. When the oil level is too high, planetary gears and other rotating parts become partially immersed in the tank oil, churning the oil and causing bubbles to form. If the oil is too low, the pump will draw in air, so that the oil and air mix, producing bubbles. If the bubbles enter the hydraulic control system, the pressure of the hydraulic control system will drop, affecting the normal work of the tank, and even slipping. And the bubbles will also cause the oil surface to rise, leading to the oil spill from the tank vent and refueling port, causing the wrong judgment.