This Delta fan(240cfm) being used as the pull fan lowered temps from 49 to 40:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835213001The problem is:
- cost
- uses 25w
- noise
- buying adapters for 4 pin
I tried a basic 120mm fan and it made no difference. There must be something between the delta and something more reasonable that will work.
NMB-MAT 4715KL-04W-B40/50:
http://www.nmbtc.com/pdf/dcfans/4715kl.pdf.
A few of my ants were shipped from bitmain with these fans (which had obviously seen significant 'burn-in test" time..) before they switched over to cheaper/more available alternatives.
Personally, I find that these things are a perfect balance between noise and cooling capability. In general fan noise doesn't both me; it's the resonance and/or squeal that some exhibit - I've never had a 4715KL which has this particular problem. Loud yes, but tolerable.
Unfortunately it can be difficult to source PWM capable versions of these fans - eBay usually has them for sale by scavengers who loot various retired IT appliances (e.g. PSUs, servers, UPS) that make use of this fan. Not for cheap either - as they seem to be in high demand.
Digikey stocks the standard 3 wire version, and again not for cheap