Answer: he has experienced Sensory adaptation
Explanation: Sensory adaptation is the reduction or partial elimination of sensitivity after prolonged or continuous exposure to a particular stimulus. This includes sense of sight, smell, hearing, touch and propioception (sense of self orientation).
Holden's sense of hearing has adapted to the high pitched sound due to exposure over a long period ot time. Sensory adaptation is important as it alerts our other senses to perceive changes around us. This has allowed Holden to be able to perform his duties as he barely notices the sound anymore.