A humming or whining noise from the gearbox at highway speed can come from the transmission, transfer case, or differential. Here’s how a Mercedes specialist narrows it down and when to stop driving.
First, figure out whether it’s really the gearbox
On a Mercedes-Benz GLS X167, a hum or whine at highway speed is a classic “follow the speed” symptom, but it doesn’t automatically mean the transmission itself is failing. The noise may come from the 9G-Tronic transmission, the 4MATIC transfer case, the rear or front differential, a wheel bearing, tires, or even a driveshaft center bearing. A good first step is a careful road test: note whether the pitch rises with engine RPM in a fixed gear or with road speed. If it tracks road speed across several gears and in neutral while coasting, the final drive, transfer case, or a bearing is more likely. If it tracks engine RPM and changes when the torque converter locks or shifts, the transmission is more suspect. AMG models and different model years can use different hardware, so confirm the exact transmission and driveline in your GLS.
