Hi,
I’m using sounds from Sharing my Music and Sound Effects - Over 1500 Tracks
All my sounds loop seamlessly, BUT I had to edit all of them with Audacity to crop the ends and get a clean loop. Audacity allows to zoom in… a lot! I told the music maker @Eric_Matyas about it, but to him, his loops are fine. 
So, my tip is get Audacity, open your file, and check/fix the loops inside Audacity.
Cheers
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