How do i stop the plopping noise when i need in poop in a public toilet?

Try flushing the toilet right when the poop hits the water, no one can tell that it even happened.

A2 you could try adding a few sheets of toilet paper to the water surface beforehand.

Try coughing or tapping your feet to distract the noise or at least channel it off a bit, but don't worry no body really cares everybody does that.