Slight vibration at steady speed between 40 and 50 mph

A light vibration that appears only at steady cruising on light throttle, and disappears when you press the accelerator, is something to describe carefully to your shop. Several things can cause it: engines with cylinder deactivation can feel slightly different while running on fewer cylinders, worn engine mounts can transmit more of that feeling into the cabin, and a torque converter or driveline issue can sometimes show up at a steady light load.

Because the tires were just balanced and it changes with throttle rather than speed, the engine and transmission side is a reasonable place to start.

A technician can road test the car, check for stored codes, look for software updates and service bulletins by VIN, and inspect the engine and transmission mounts. Note the speed, gear, and whether it happens when the engine is cold or warm.