[banditpowdercoat]

Well, sounds for starters, it's plumbed wrong, a bit You Should have a valve on the return pump line to tank. You leave the Tank drain to sump Open, unless you are working on it and need to shut off for some reason. But you control the flow with the Return pump valve. Also, what kind of drain do you have in your overflow? Just a single line from tank to sump? Durso drain in overflow then, or just a pure open pipe? Pure open pipe will sound like a toilet flushing. You need some kind of siphon break like a Durso, etc.

You should NEVER throttle your drain back if there is only one drain. Herbie style drains throttle the main pipe but have a completely un-restricted emergency drain in case the main plugs up.