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.

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

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.

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
 }

When it goes to audio, always do a double check in a DAW before exporting to an Engine. It's very important to find out where the problem is. Sometimes it's a problem with the sound wave.

First of all, try looping one of the sounds all alone. Since you are using an uncompressed AIFF sound there won't be a looping problem. Them do the same with the second one.

Second, try adding some silence to the end of the clip. Since you are using two Audio Sources it won't cause gaps between sounds

Third, you can try getting the sound exported in other formats, like .mp3 and .wav.

I hope one of this solutions solves your problem.