I have a new sandy bridge system. My case is an Antec P180B.
Right now I have no case fans accept the Noctua heat sink fans (and PSU fan). The Noctuas seamed really noisy at first (compared to my old Nexus D12SL-12 fans). So I put the ULNA voltage adapters on them and they got quieter.
Ultimately I'm looking for a really quiet machine. I don't do a lot of high intensity stuff on this machine but when I do I would like it to stay cool.
I'm thinking of several fan configurations:
1) Leaving it the way it is now. No case fan but both Noctua fans on low voltage NH-D14.
2) Removing the outside Noctua NH-D14 120MM fan from the heat sink (and leaving 140 MM in the middle of the heat sink).
3) Removing the outside Noctua NH-D14 120MM fan from the heat sink, leaving 140 MM in the middle of the heat sink). Plus adding a Nexus D12 (120mm) mounted to the case but right next to the heat sink.
I have no idea, personally (sorry!), but you might try posting this question in the Noctua's manufacturer's support forum, and see if you get any hits there (assuming you don't get a good response here).