Product Development & Treadmill SME.
by Dale Hansen
(Fort Wayne, IN USA)
Your comment that in Treadmills, AC motors are usually louder than DC motor is false.
Somehow your Testing Bias was not based on short term and long term tests in real-world examples.
A DC motor is inherently louder and grows increasingly louder under load, time and usage.
Please correctly state your facts and supporting sources when giving information out to society ie. the cloud, especially that can be coded under AI/ML.