BigS brought up a good question in another thread about the cause of noisy flywheels. There are a couple of common problems. For instance, the cage might not be screwed down tightly, allowing it to vibrate against the shell. Another issue is loose bushings/bearings in the motor. Unfortunately there's not much you can do about that except replace the motor.
But the most common problem is off-center, and imbalanced flywheels. Stock flywheels are notorious for being cheaply made and having poor quality control. Even aftermarket flywheels are sometimes out of balance. Fortunately, there's something simple you can do about it IF you're willing to disassemble the flywheel cage and pull off the flywheels.
Flywheel centerline
When you power down, there will be a more significant mark on the heavy side of the flywheel. A short mark means the flywheels are way out of balance. If the line goes all the way around but it's thinner/lighter on one side and wider/darker on the other, the wider/darker side is the heavy side. If the line is even all the way around, the flywheel is in balance. Check both wheels the same way.
Remove the imbalanced flywheel using the two-screwdriver method, and put a piece of electrical tape or duct tape on the lighter side of the flywheel. If the mark is short, stack a couple of half inch pieces on top of each other to get some weight on the other side. If the mark is long or goes all the way around the rim, it means the flywheel is just slightly out of balance. Put a single half inch piece of tape on the light side. Reassemble and check it again.
It might take a couple times, but eventually you'll get to a point where the vibration is tolerable and the noise is greatly reduced.
