Is it the belt jumping ?
If so, you need to check
- The belt tension (top of belt should be tight enough so light hand pressure from above does not make it contact top tube) … adjust by slackening and rotating the eccentric BB, untill tighter … BUT not too tight which wears out everything faster.
- Snubber has the right gap (small as possible without touching the back of the belt … ie about 0.25mm) … adjust by slackening lock nut then snubber bolt, to allow the bearing position to be adjusted, then re-tighten. I find this takes a few attempts as the position seem to move as the bolt and locknut are tightened. There is plenty of info on this forum … search ‘Snubber’
If its not the belt jumping … I’m not so sure what it could be, might need some more careful listening and diagnosing.