How to get fresh air to flat without letting noise in
Firstly, it may be good to at least consult an HVAC specialist regarding any code implications to vents and air return modification.

From a logistics standpoint, there are several manufacturers of 'silent' centripetal inline  fans made to mate to 4" round ducting on either side.

Pair this fan with a electronically controlled damper and some exterior access with a baffle/squirrel cage and some 4" round ducting and you should be able to force fresh air into a room, or series of rooms. One might be careful to allow access to fresh air, but without allowing in all of the other elements; Hence my suggestion for an electric controlled baffle that would prevent extremely cold or hot air from passively infiltrating when the system was not on.

Since you aren't necessarily concerned with heating or cooling the room, total CFM requirements may be a lower priority than distance/degrees of separation from inside to outside. If the inline fan was remotely located near the source of the fresh air, and the run was long enough, I would be confident that you could source the air, without all the noise pollution.

TL:DR

Exterior Weather Resistance Inlet -> Electric controlled damper -> Inline Fan -> necessary Ducting -> Interior Vent