Re: introduction/questions for the experts
Switch up to 32:1. You will be pleased.
If your chain is slapping a lot, it's too loose, or the rear sprocket is out of alignment, or both. Standing still, with the top of the chain run tight, the bottom of the chain run should have up to about 3/4" of play. Pay close attention to the rear sprocket alignment by removing the rear wheel, holding it vertically by the axle on each side, and spinning it while you hold it. Look edgelong at the spinning sprocket, and you'll see quickly if it's off center (bumping up and down), or off kilter (wobbling left and right). Adjust it as necessary.
Occasionally, the sprocket could be warped. You might be able to sort that out by removing the sprocket and working it gently with a hammer on a block of wood, but you might also destroy it completely.