Letting fresh air through a window without letting noise in
Short of replacing your window with a duct with several turns, lined with sound baffles, you aren't going to get quiet.

Human ear is amazingly sensitive.  30 db is a very quiet country side.  Flies buzzing, birds chirping.  Office is about 70 db  40 db difference doesn't sound like much, but it's a log scale.  That office is a thousand times noisier.

3 db is a factor of 2 in noise.  That is detectable as being quieter.  Just.

To make a meaningful difference you have to take at least 10 db (factor of 10) to 12 db (factor of 16)  This is difficult.

Windows that are designed to reduce sound are made of heavy glass, are double pane, and one pane is thicker than the other, they are mounted in a way that isolates them from each other and from the frame.  

Ducts that reduce noise transmission are lined with irregular chunks of foam, and have several corners with T's so that the air has to corner while the sound goes straight.  Hard to do well.

More information:  Research sound stage building design.



What you can do is mask it.  Get a set of external speakers you can plug into your phone, and get a recording of surf, or babbling brook, or wind in the pines.