The gearbox mount (transmission mount) secures the gearbox to the vehicle's subframe or chassis and absorbs vibration and torque reaction. A worn or collapsed mount allows excessive gearbox movement, causing clunking on gear changes, vibration, and potential damage to driveshafts, exhaust, and shift linkages.
Gearbox mounts contain rubber elements that deteriorate with age, oil contamination, and vibration. Hydraulic mounts add fluid damping and are more complex.
Inspect gearbox mounts by rocking the gearbox with a lever. Excessive movement or torn rubber indicates replacement is needed.