“Donk” sound coming from cassette when under pressure

I'm not really sure how to explain this in the best way, but whenever I pedal or make the cassette move forward in any way, there is a "donk" sound and it feels like my pedals are loose when that happens, even though they are not. I took off the rear wheel to inspect the cassette more closely, and when I rotate the cassette clockwise, it still rotates a bit and there is that same "donk" sound