Yeah, this one's common. It's normal. The chirping or buzzing is just the sound of large particles passing through the PlasmaWave technology. It doesn't mean the product is failing.
If the noise is more of an abnormal vibration or rattle, that's different. Make sure the unit is on a hard, flat surface. A flimsy or slanted surface can cause noise. Also, check that the front panel is latched securely closed and that nothing is blocking the air intake or outlet vents.
If that didn't work: Ensure you have at least 12 to 18 inches of clearance from walls and other electronics like TVs or radios. Electromagnetic interference can cause malfunctions.
Heads up: Never insert fingers or foreign objects into the vents to try and investigate the noise.