I can't hear my Noctuas at all in the server, I have to say. I can now hear this temporary Cooler Master I put in place, but not that much.

I also do the same: big and slow fans. Noctua usually includes adapters to slow down fans, as you did, down to 900-800-700 rpm, depending on the models. I do slow them down all the times, without temp issues. I've been quite happy with Noctua so far.

On my main PC, though, which was built to be a gaming PC (in the hope I had time to play games) I installed more Noctua fans than actually needed (purpose: overclock, just for fun, and getting rid of the noisy video card fans, completely). The PC is not "silent", but, to be accurate, I should say that I do no hear the fans themselves, but rather the massive airflow in and out of the case. I would still consider it "quiet" compared to any other similarly built PC I owned in the past.