Fully electric helicopters would be much more silent indeed: turbines are incredibly noisy, and the gearbox is another source of noise that would not be present since electric motors don't require one. Use a many bladed rotor with swept tips and high aspect ratio blades, and rotor noise would be reduced as well.
They would also have a short range and a small payload capacity, since a lot of it is used for lugging batteries around. Weight is a serious issue for aircraft in general and helicopters in particular: vertical flight can use very little aerodynamic L/D leverage effect. In vertical flight, thrust = weight.
A hybrid helicopter would reduce battery size: a turbine plus generator provides electricity for the electric motor and chargess the battery. When within range of the target, the turbine is shut down and the helicopter continues forward flight and landing on the batteries. Extra weight compared to conventional would be the electric motor + generator + batteries, weight saving = transmission. Eject the spent batteries on the way home for increased range.