What Causes Abnormal Noises in the Transmission?

[CharlotteFitz]

Transmission gear noise is usually caused by low or deteriorated transmission fluid level, excessive or insufficient gear meshing clearance, or poor gear meshing. When diagnosing the cause of transmission noise, it is important to first identify the pattern of the noise and further determine possible causes and solutions based on the situation. Below is relevant information about transmission issues: 1. When the gear meshing clearance is too large, the driving gear impacts the driven gear during rotation, producing a knocking sound. This noise typically worsens as the transmission temperature increases. When the meshing clearance is too small, the abnormal noise is usually continuous—sometimes uniform, sometimes irregular—and becomes more severe with higher vehicle speed, often causing the transmission lubricant to overheat. Gear noise can also result from damaged gear tooth surfaces, producing an irregular, rhythmic noise that intensifies with greater throttle input. In severe cases, vibrations in the transmission may be felt. 2. If abnormal noise is heard from the transmission area in the cabin during idle or driving, possible causes include: insufficient or deteriorated transmission fluid; worn, loose, or damaged transmission bearings; bent transmission shafts; abnormal gear meshing with excessive or insufficient clearance; metal peeling or broken gear teeth, or misalignment during repair or assembly; or looseness between the gearshift lever and the shift fork groove. Corresponding measures: If there is a metallic dry friction noise while driving and the transmission housing feels hot to the touch, this is likely due to insufficient or deteriorated lubricant. The fluid should be topped up or inspected, and replaced if necessary. 3. If abnormal noise occurs in neutral but disappears when the clutch pedal is depressed, it is usually due to wear or looseness in the front or rear bearings of the transmission input shaft, or noise from the constant-mesh gears. If the noise persists in all gears, it is often caused by the rear bearing of the output shaft. Severely loose or damaged bearings should be repaired or replaced.