My machine is two years old, and is much louder than it was originally. I took it to my DME. He took off the cover of the base unit, to show me the sealed motor. He explained that over time, minute dust particles can accumulate in there. When the motor spins, the dust particles get between the fan and the housing, causing the noise I have noticed. Since it's a sealed motor, they can't open it to clean it, so I'm stuck with the noise.
If your machine is new, this shouldn't be an issue. But if it was used, it could be.