Causes of noise in a manual and automatic transmission

When a car’s transmission goes bad, whether it is manual or automatic, one of the symptoms that tell you that the transmission is failing are the noises that it emits. On a manual car, the sound is like a hum because of worn bearings, shaft gears, differential.

In an automatic transmission, you will often hear a buzzing because of low oil levels and problems with the torque converter and the lever arm.

The first thing you should check when you hear noises in a manual or automatic transmission is the oil level. If the oil level is not enough, you have to add more oil until it reaches the desired level, change it entirely, or check the seals and gaskets if you suspect a leak.

The fluid level in the transmission is checked more often in an automatic transmission because, you still, at some point, have to change the oil at a certain number of miles 60.000 km – 70.000 km (37.000 miles – 43.000 miles). You could use a temporary solution like a good transmission additive. It will help by reducing the hum and buzzing intensity, but it won’t reduce it completely. For that, you need to remove the transmission and check the problem in depth.

Why does the car transmission make noise?

Low level of transmission fluid

Transmission fluid is the most common cause of noise in transmission. In rare cases in manual transmission and often in an automatic transmission, the low oil and the fact that it is losing its properties over time are responsible for the noises in the car transmission.

The noises that the transmission makes because of the fluid resembles a metallic clang, which is louder as the car’s speed increases. If its level is insufficient, then as a result the bearings and other moving parts of the box will run dry, emitting a loud noise. 

This is not only uncomfortable while driving, but also dangerous to those parts. Therefore, it is always necessary to monitor the gearbox oil level and its viscosity.

Because of the low oil level in transmission, the noise is heard when:

  • The engine is still cold. Also here, the cause of the hum may lie in the thickness of the fluid and its contamination.
  • The car is moving, and the noise increases in intensity as the car’s speed increases;
  • The engine is idle, or the gear is in neutral;

Damaged driveshaft bearings

Another common cause of the noise is bad or worn drive shaft bearings. This happens due to natural wear and tear, a small amount of lubricant in them, or dirt that has accumulated inside these bearings. The noise can be heard:

  1. when the engine is running at high speeds;
  2. when starting the car and the engine is cold, the noise can be present in the engine at low speeds then it goes away.
  3. when driving on a hill;
  4. when the car is moving, but the hum disappears when the clutch is pressed
  5. the noise is quieter at low speed and louder when you increase the speed.

Regarding the first 3 situations, they can occur if the bearing from the first drive shaft is damaged, and the other two when the bearings from the second driveshaft are worn.

Bad or worn gear synchros

Another cause of hum coming from the transmission is because of the wear of the gear synchronizers. Sounding like a metallic clang, the intensity increases as the engine speed goes up.

When the synchros are bad, you will notice that changing gears will be difficult in a manual transmission, and also the gear shifter will have a hard time remaining in the selected gear and will set itself to neutral.

If the manual transmission makes a noise at idle when the clutch is released, and when the car is stationary, then most likely the bearing on the input shaft is making noise. 

If the manual transmission hums more in first or second gear, then a large load goes to the front bearings. So, you have to check the input shaft bearing.

The input shaft bearing can make noise when the car is on a hill or just after starting the engine, no matter the gear position. Often the noise disappears when the clutch is pressed. 

The reason for this is that when the clutch is pressed, the primary does not spin, and the bearing also does not spin.

If the transmission is noisy in 4th or 5th gear, then in this case a large load goes to the rear bearings, on the secondary shaft. These bearings can also make noise in any gear, including reverse. The higher the gear position, the louder the hum. In the fifth gear, the intensity of the hum will be maximum.

A failing torque converter

In an automatic transmission, the source of noise can be coming from a bad torque converter. Usually, the hum from the torque converter is heard when changing gears at low speed, as the driving speed goes up, the sounds disappear, after 37 miles/h.

Additional signs of a bad torque converter

  • The car jerks and vibrates when moving;
  • The car is slipping when starting;
  • You can feel the smell of burning in the cabin;
  • Wear of individual friction discs
  • RPM does not rise after certain values;
  • Depressurization due to destruction of oil seals;
  • Wear of intermediate and support bearings;
  • Low transmission fluid level.

Differential gears

Another reason for the transmission noise can be the wear of the differential gears. The sound they make resembles a metallic noise. It usually appears smoothly and is not noticed by the driver, but you can see this symptom when the car slips. In this case, the drive wheels rotate unevenly, but with large torque. This puts significant stress on the differential, and it will fail faster.

The best way to notice the failing differential is when you start the engine, the car is rolling back and forth. If you see this, it’s best to check the condition of the differential.

Transmission noise additives

Additives can help reduce the transmission noise, but they will not help solve the problem. They are a temporary solution for noise reduction and can slow the damaging process.

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