Check out the condition of ozone hose from the ozone generator to the injectors. There may developed some cracks or holes. Also there are chances for the black tubing to get malfunctioned. If there is some issues, it will suck in additional air, and that can contribute to the noise you are hearing.
I also had a similar issue with my hot tub years back. I used standard vinyl tubing to replace it with. It has given us better service than you would expect: some have been running with it now for five years. It turns brown inside, but it seems to stay in place and do the job. You may need to put the ends in a cup of hot water to get them to ease onto the barbed fittings and the check valve.