Why does the gearbox knock in manual and automatic transmissions?

Anna Tkach

AutoPortal editorial team

437 views 3 min read

Gearbox knocking is one of the common problems faced by owners of cars with manual and automatic transmissions. Characteristic sounds can occur at idle, when shifting gears, or while driving. To understand why the gearbox is knocking, it is important to consider the type of gearbox, operating conditions, and the nature of the noises themselves.

Causes of knocking in a manual gearbox

When a manual gearbox knocks, it can signal both minor issues and serious breakdowns. Below are the main causes of knocking:

Wear of gears and synchronizers

Over time, the teeth of gears and synchronizers in a manual gearbox wear out. As a result, a characteristic knocking occurs when shifting gears. If the gearbox knocks when shifting, especially when upshifting or downshifting, it is recommended to check the condition of these elements.

Insufficient level or poor quality of oil

Lack of transmission oil or its poor quality is another common reason why a manual gearbox knocks. This is especially noticeable at idle speed. The oil loses its properties, friction increases, and knocking appears.

Wear of bearings

Bearings ensure stable operation of the shafts. Their wear leads to the appearance of noises and knocks. This is especially noticeable at idle or during sharp starts.

Causes of knocking in an automatic gearbox

If an automatic gearbox knocks, the nature of the malfunctions differs from a manual one. Here are the main causes:

Problems with the torque converter

The torque converter transmits torque from the engine to the transmission. Its failure can cause knocks when starting to move or stopping. This is accompanied by jerks and jolts.

Wear of friction discs and solenoids

Friction discs and solenoids control gear shifting. When these components wear out, the gearbox starts knocking when shifting. Delays when switching between modes are also possible.

Errors in the control system

The electronic control unit of the automatic transmission may produce errors, causing shifting to become incorrect. This leads to impacts and knocks, especially at low speeds.

The gearbox knocks at idle: what could it be?

If the gearbox knocks at idle, it may be caused not only by problems inside the transmission.

Loosening of the power unit mounts

With weakened engine or gearbox mounts, vibration is transmitted to the body, creating the impression of knocking. This is especially noticeable at idle speed.

Internal wear of gearbox elements

Even at idle, the shafts rotate inside the gearbox. If gears or bearings are worn, extraneous sounds can be heard without load.

How to understand that it is the gearbox that is knocking?

You can determine that it is the gearbox that is knocking by the following signs:

Knocking occurs when shifting - most likely, the problem is in synchronizers or friction discs.
Knocking at idle - it is worth checking the oil and engine mounts.
Knocking while driving - possibly wear of gears or shafts.

It is important to distinguish gearbox knocks from sounds of the suspension, steering, or engine.

What to do if the gearbox knocks?

At the first signs of knocking, do not postpone diagnostics:

Check the transmission oil level.
Determine the conditions under which the knocking occurs: at idle, when shifting, at a certain speed.
Contact specialists. The earlier the cause is identified, the cheaper and easier it is to eliminate it.

Knocking in the gearbox is an alarming signal. Regardless of whether your car has a manual or automatic transmission, when knocking appears, you should take action immediately. Does a manual gearbox knock when shifting? Or does the gearbox knock at idle? In any case, do not ignore such symptoms. Timely diagnostics and repair will help maintain the operability of the transmission and avoid costly consequences.