Diagnosis
This is either planetary gear wear in the transmission or a failing brushless motor bearing. The noise under load points to the gears.
How to Fix It
First, isolate the sound. Run the drill with no bit, no load, in both gear 1 and gear 2. If the noise is only present in one gear, that gear set is worn. If it's in both, it's likely the motor bearing or armature.
If That Didn't Work
For a gear noise in one speed, the transmission needs servicing. The planetary gear sets and bearings are a sealed unit. Bosch sells it as a complete assembly. It is not intended for end-user repair.
Heads Up
Do not continue using a drill making metallic grinding sounds. You will turn a gear replacement into a full transmission seizure, which can damage the motor.