What are the reasons for the loud engine noise in the Previa?

[Walker]

Carbon deposits and transmission overload operation can cause loud engine noise. Carbon deposits: Carbon deposits can cause incomplete combustion of gasoline, leading to engine shaking. Engine carbon deposits have a significant impact on the car: first, they reduce engine power, resulting in uneven and gradually diminishing power output, making the car feel increasingly sluggish. Second, they increase fuel consumption, adding to your financial burden. Third, they make cold starts difficult, meaning the engine is hard to ignite. In severe cases, combustion chamber carbon deposits can even cause cylinder knocking, producing noises during low-speed acceleration, damaging pistons and crankshafts, and leading to engine overheating, which severely compromises vehicle safety. Transmission overload operation: Transmission overload operation can degrade the quality and friction characteristics of transmission fluid, increasing friction and wear, which leads to transmission noise. At the same time, sludge (impurities or dirt) can also form inside the transmission (e.g., in the valve body), causing issues such as jerking, gear slipping, impact, slipping, abnormal noises, delays, high temperatures, and oil leaks.