Same issue, Try to change Sound Asset Compression Type to PCM, for assets that have ‘pop’ while looping, it helped me
I think it’s because of Unreal’ audio compression and how it changes (compresses) frequencies and shifts their phase. For some loops nothing happens for other you can hear ‘pop’
if you still hear ‘pop’ at the beginning of Wave but not while looping: add Enveloper and draw the Fade-In in Volume Curve