A chain drive bike will never be completely silent as you have metal on metal contact, but it should not be obnoxiously loud. The first thing to do is wipe off the old grease and apply some fresh white lithium grease. If you have had the bike for a while and it is starting to get to loud.
If the noise continues then we want to check is chain tension. Too little and or too much tension on the chain can cause noise. You want about ½ inch of defection when you pinch the mid-section of the chain (pointed out below).
To adjust tension on the chain we will be working with the flywheel side. After loosening up the flywheel axles securing nuts (pointed out in red). To only affect the tension on the chain you want to rotate both alignment bolt nuts (circled in green) EQUALLY in the same direction. Tightening adds tension and loosening will lessen tension.
If your chain is still putting off excess noise, then we want to adjust the angle on that flywheel. You will do this by rotating each alignment bolt nut (circled in green) in opposite directions. Do a 1/4 rotation on each side and then spin the flywheel to see if that changed the noise at all then repeat as needed. If the noise gets worse in the direction you are working, reverse your rotations and make the flywheel angle in the other direction.