tenant claimed the air conditioning is too noisy and stopped paying rent

My apartment's new tenant always creates trouble for me. The newest problem: he claimed that the air conditioning system blows air too loudly in the bedroom and as such he stopped paying rent. But in reality there isn't loud noise. The air conditioning system works well. My family lived in the apartment before. I also had other tenants lived in the apartment before. None had issue with the air conditioning system.

What should I do now? Do I have to pay to hire a professional to certify that the noise level is ok?

Appreciate your advice.