Why Doesn't the Differential Hum at Low Speeds?

[PaigeAnn]

Differential humming at low speeds mainly has the following reasons: Insufficient Lubricating Oil: If the differential lacks lubricating oil or the quality of the oil is poor, it can easily cause abnormal noise from the differential. In this case, simply replenishing the lubricating oil will suffice. Worn Differential Gears: If the gears are worn, they cannot properly mesh with other components during operation, causing friction between parts and leading to abnormal noise from the differential. In this situation, the gears need to be replaced. Improper Gear Adjustment: Similarly, if the spacing between the gears is not properly adjusted, it can also cause abnormal noise. In this case, the spacing needs to be readjusted. Foreign Objects in the Differential: When foreign objects such as mud or sand get stuck in the differential, they can also cause abnormal noise.

[LaBarrett]

The phenomenon where a differential remains quiet at low speeds but produces a humming noise at high speeds is fundamentally related to the meshing state of the gears. The spiral bevel gears inside the differential develop play after wear. At low speeds, the impact force from gear meshing is minimal, making the vibration caused by the play unnoticeable—similar to how gently closing a drawer doesn’t produce a clunking sound. However, as speed increases, both the rotational speed and force on the gears grow exponentially, amplifying the tiny play into high-frequency vibrations through repeated impacts. Additionally, at high speeds, the driveshaft forms a specific resonance frequency, and the sound is magnified through the metal structure of the chassis, making the humming noise particularly pronounced. Furthermore, aged differential oil exacerbates wear, as the breakdown of the oil film leads to direct metal-on-metal friction. This dry friction noise becomes more apparent at high speeds.

[MacRichard]

I often encounter this phenomenon when repairing cars. The speed range of differential noise is crucial: if it doesn't make noise at low speeds, it means the gears haven't reached the resonance point, just like when playing the violin, the string won't make a sound if the bow speed is insufficient. After the gear pair wears out, pits and steps will appear on the tooth surface. At low speeds, the oil film between the gear teeth still provides a buffer, allowing the teeth to 'slide' over the worn points. However, when the speed exceeds 80km/h, the rapid increase in rotation speed prevents the oil film from filling the gaps in time, and each impact of the gear teeth is like hitting a steel pipe with a hammer. At the same time, the differential housing resonates like a speaker cabinet at high speeds, transforming the impact noise into a continuous buzzing sound. If a new car makes noise, it may be due to improper installation, while for older cars, it's mostly natural wear and tear.

[LeColeman]

Simply put, worn gears need to reach a rotational speed threshold to produce noise. The differential's internal planetary gears and axle shafts have extremely high precision alignment. After prolonged use, they develop wear at the 0.1mm level. At low speeds, the slow rotation rhythm means collision energy from wear points is negligible—like slowly rubbing a toothpick won't break it, but rapid rubbing easily snaps it. When wheel speed exceeds a critical value (typically 60-80km/h), impact energy grows exponentially. Metal fatigue accumulates, generating high-frequency sound waves. At this point, the damping effect of lubricant weakens, allowing noise to transmit directly through the rigid rear axle into the cabin. This is why older cars often make seats vibrate noticeably at highway speeds.

[AmyMarie]

The key lies in the amplitude and frequency difference. Worn differentials generate high-frequency vibration sources, but the amplitude is too weak at low speeds. Taking a household car's rear axle as an example: the vibration frequency at low speeds is around 200Hz, which is less perceptible to the human ear. However, at high speeds, the vibration frequency exceeds 600Hz, entering the frequency range most sensitive to human hearing. More critically, tires and drive shafts generate low-frequency vibrations of 9-13Hz at 80km/h. The superposition of these two types of vibrations creates modulated resonance. It's like lightly tapping a bell produces no sound, but striking it hard resonates throughout the valley. Therefore, it's recommended to replace differential oil every 50,000 kilometers, as oxidized oil loses its damping effect.