Yeah, this one's common. It's usually the air compressor or oxygen concentrator, not the ozone generator itself. Those are separate required accessories that pump air and supply oxygen feed gas to the cell.
First, confirm the noise is coming from your Absolute Ozone unit and not the accessory equipment. If it's definitely the generator, the internal iOzone controller protects against problems that could cause noise, like bad feed gas quality (too much humidity or contamination) or incorrect operating pressure.
If that didn't work: Excessive pressure fluctuations can damage the electronics and cell, potentially causing noise. For applications where pressure fluctuations are possible, you need a buffer tank with a hi-low pressure switch installed. Also, ensure all gas connection tubing is ozone and oxygen-appropriate material and connections are tight.
Heads up: Do not obstruct the cooling air vents on the enclosure. Overheating can cause the cooling system to fail and the controller to shut things down.