Overflow / Sump Issue

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My Overflow / Sump isn't working properly and I just can't figure out why.

The issue is water flow, my Overflow will suck the water to my sump, then make a loud Gurgle noise. The flow of water goes from super fast in my sump, to trickling slow. This happens every 15 seconds...the noise is very annoying.

Any ideas what would cause this issue and how to fix it?