I simply cannot understand how a helicopter can be made quieter. What are the things that would considerably affect the noise signature of a helicopter ?
I'm no expert but the RAH-66 Comanche can help answer your question.
It was a stealth attack helicopter prototype.
The Wikipedia page lists two main design elements:
A five bladed rotor, the more blades you have, the slower they'll turn, so less noise. Of course they are harder to balance, so it's not cheap.
Canted blades, kind of like winglets. They reduce the blade-tip vortices. The intricate shaping also is expensive, as the blades were 100% composite.
Sound proofing the engine compartment and muffling the exhaust can be expected as well. But that's added weight.