Larger mixers can be louder than smaller capacity mixers, due to the need for a larger motor and all-metal gears. They may sound shrill or "airplane-like" DC motor mixers are quieter than AC model mixers.
If you are used to a smaller mixer and then purchase a larger one, it may seem loud, initially. There are individual sound variations even among the same model mixer.
Is the mixer clicking?
Clicking during heavy mixing is not unusual, especially while kneading yeast bread. Clicking may be more pronounced in larger 7 and 8-quart mixers because they have DC motors which are quieter.