How to turn the auto focus sound off for nikons?

I have a Nikon D5500

Added (1). I meant the sound the camera makes once it focuses in on a target and makes a "Beep" as described in the best answer.

Perhaps you're talking about the beep noise when your camera attains focus lock. In your manual, look in the index for "Beep". It might be d1 in your camera's menu system.

If it's AF motor noise, don't use auto focus when recording videos. Use manual focus or better yet, use an external mic or recording device. The sound is mechanical and part of the lens. If you want a silent focusing camera, you will have to either manually focus your current lens, or sell it and get one with a silent motor. Turn camera off.

OR

Switch to MF.

Seriously! The "sound" is the sound of the auto-focus motor in the lens doing its job.

Did you think that it was an artificial sound like the shutter sound on mobiles? Not possible. It can't be turned off and on. Unfortunately, it's one of the issues with using a DSLR for video recording, if that's the problem you're having.

The sound is mechanical and part of the lens. If you want a silent focusing camera, you will have to either manually focus your current lens, or sell it and get one with a silent motor.

Turn camera off.

OR

Switch to MF.

The "sound" is the sound of the auto-focus motor in the lens doing its job.

Did you think that it was an artificial sound like the shutter sound on mobiles?

Not possible. It can't be turned off and on. Unfortunately, it's one of the issues with using a DSLR for video recording, if that's the problem you're having.