anybody tried part cooling fans which don’t whistle as well are effective at cooling?
most of the fan shrouds i tried make a horrible whistling sound when they are at full blast… the ones that are silent don’t give enough cooling!
Might have to take a lesson from the auto industry. Instead of eliminating the noise the best you may be able to do is tune it so it isn’t as annoying. Different nozzle shapes may tune the whistle.
Consider adapting one of these high efficiency duct styles. Uses a 50mm high static pressure centrifugal fan.
or
A gradually reducing area duct design that provides a straight shot of air with no noise inducing turns and 90 degree impacts in-between.
Quiet even at full blast for me. Very effective when the duct tip slope is pointing right at or just above the nozzle tip. Blows a concentrated stream of air right at the tip just about horizontal to the print. I can bridge 15mm with less than 0.1mm of sag and have done as long as 96mm of bridge with less than 0.3mm of sag. I can also do 5mm horizontal overhangs 3mm wide and they are flat by layer 4 (0.1mm layers)
See this torture test part for results and future testing.
The only whistle I get is when the part geometry is just right it whistles from the air passing over a hole. For me it is just part of the quiet song my printer sings while printing. 
The blower fan will be noisier than the air stream produced and with a little Teflon lube even that is quiet…