Typically, this kind of thing points at issues with the source asset, usually with the format they’re in. MP3 being the source format is a common culprit, for reasons you can read here: MP3 loops with an audible gap, the equivalent WAV doesn't - #3 by Leah_FMOD
If your source assets are MP3s, I’d recommend re-rendering or re-encoding to another format like WAV or OGG. If they aren’t, what format are you using?