What causes the audio clicking & how do I get a clean sound?

I had the same issue with PCM compression format. When I chaneged "Load Type" to "Streaming", problem was solved. "Compressed in memory" or "Decompress on load" was causing the clicking sounds. Also Compression format ADPCM or Vorbis worked for me,

This can be set directly in Inspector in Unity when clicking on audio file.

I realize this is an old thread but I wanted to share how I solved this issue. I had a similar problem with some audio clips I had and the fixes listed here did not work for me. I solved my issue by exporting at a higher bitrate. Originally they were 44100 (Hz) but changing to 48000 (Hz) and exporting as a .wav file solved the clicking noise issue.

Go to ProjectSettings>Audio and set the DSP buffer size to best performance. it works.

The clicking comes from not starting or ending at a zero sample... and when shutting off a looping sound or any sound anywhere before the end, you could do the fade out your self like this

 AudioSource oASEngine;
 bool bEngineFadeOut = false;
 ...
 
 void Start()
 {
     ...
     foreach (AudioSource aSource in GetComponents<AudioSource>()) {
         if (aSource.clip!=null && aSource.clip.name.Equals("engine"))
             oASEngine = aSource;
         else ...
     }
 }
 
 void FixedUpdate()
 {
     ...
     if (startEngine) {
         bEngineFadeOut = false;
         oASEngine.volume = 1.0f;
         oASEngine.Play();
     } else if(stopEngine) {
         //oASEngine.Pause(); //don't, this may cause clicking noise
         bEngineFadeOut = true;
     }
     if(bEngineFadeOut)
         oASEngine.volume *= 0.8f; //fade out over ~100-200 ms
 }

I just had this problem. I switched up some audio to use 3D (It's a 2D shooter) and I wanted more spatial awareness in my clips. I drop recyclable objects for explosions and audio, and after switching to 3D, I got clicks. This mostly happened on recycling the pooled object... for my situation, it ended up being a timing issue... delaying the playback until into the second Update call solved the issue nicely. Since the 3D call depends on the placement of the gameobject, and a rapid change in that position can skew the playback, I suspect that was resulting in the click. TL;DR version... Delay playing the clip back on 3D audio until position has been established.