Q: Truck making loud gear noise when driving

This type of noise is typical of a bad hub bearing. When you turn left, you put more pressure on the right tire stressing the already bad bearing. This is why the noise seems to get louder. Bad hub bearings can separate causing the tire and part of the hub to come off the vehicle. This can be dangerous as it typically happens while driving. I suggest getting it checked out by a qualified mechanic who can verify this is the cause of the noise. The Check Engine Light will be a separate issue in this case and the mechanic will be able to scan the vehicle to extract the code. If you have noticed any drivability issues besides the noise I would mention this to the mechanic as well as it can help with the diagnosis.