When the fan is spinning at a constant speed, the noise level should be low. When you switch from inhaling to exhaling, the motor will ramp down, and then up again as you inhale. If you have EPR enabled (or if you are on bilevel), the motor will spin down even more when you exhale, meaning it will have to work harder to spin back up when you inhale. This can cause more noise.
Much like in software architecture, everything in CPAP is a tradeoff. High pressures can cause mask leaks and excessive farting. Low pressures can be more comfortable but can cause snoring or apneas. EPR can cause noise and possibly reduce efficacy. Heated humidification can improve comfort, but requires more maintenance on mask/hose/tank. One mask make be more quiet, but may start to leak as you roll over. Etc.
And much like software projects, every patient is slightly different. Finding out what works isn't easy. Fortunately, you have the rest of your life to optimize therapy. And the better you optimize, the longer that life will be.
The perfect mask for me is the nose cushion. N30i for me, if anyone is curious.
The noise doesn't bother me much. By the time it ramps up I'm asleep.