The designs made in the 90's were relatively inefficient which is one reason why they're noisy. But at the same time: it's impressive that a turbine from those days is still up and running today and that says something about the engineering that went into that thing. Lots of lessons have been learned and a modern turbine is so quiet you can stand right underneath it and barely hear it, which I find absolutely incredible. A 75 meter diameter rotor intuitively should make a lot of noise, but they really don't. The main driver is the precision with which the filament is laid on the blade was well as how close the back edge of the blade approaches zero. That's the biggest source of noise and you'll always have some either because you don't quite get there or because you do and the back edge starts to flutter. It is one of those very annoying engineering trade-offs where you're damned if you do and damned if you don't.

Work out the tip speed on one of those big machine and you'll be even more impressed.