It is impossible for any kind of car to become louder after maintenance. This is abnormal. This statement of the 4S store can only be explained by perfunctory.

The only reason for the louder sound is that there is a large gap in the internal bearing of the rear axle or it has been burned out. No matter how poor the oil is used between gears, there will be no loud noise. Apart from the bearings, there are only gears inside the rear axle. "As a reminder, check the source of the sound carefully. Whether it is the abnormal sound coming from the tire bearings, but in short, you can go to the 4S store to deal with it. If it doesn’t work, just return the car.”

I suggest you negotiate with the 4S store. If it doesn’t help you solve it, call 12315. If that doesn't work, ask for the car to be returned.