I've already looked at the Scythe Mini Kaze Ultra but at just under 20dB for less then 5cfm it's way to noisy
Only thing I've found so far is this Noctua http://www.heatsinkfactory.com/noctua-nfa4x10flx.html
Good Airflow with the adapter used 6.6cfm @ 12.9dB or 8.2cfm @ 18+dB w/o the adapterProviding 3000, 2400 and 1600 rpm speed settings via the supplied Low-Noise and Ultra-Low-Noise Adapters, the NF-A6x25 FLX (Flexibility) can be fine-tuned for superior airflow or maximum quietness.
Specs show that 12.9 is at 3700 with the adapter. Wonder if it's even quieter with the Ultra Low Noise adapter?