Need more info really, what type of fans are we talking? If you're a 'grow your own' fanatic then the S&P ducting fans are pretty good. Ultimately all fans will be audible unless run through ducting and baffles/chambers or over distance.
Its likely that the problem is related to there being no separation between the fan unit and the wall and the walls are accentuating the noise levels created by the fans.
A solution would be to get a material known as 'regupole' which is used for acoustic insulation, around 6mm thickness will do and cut out a gasket to act as an interface between the back of the fan housing and the wall. Also as per earlier thread take the fan out and give it a clean including oiling bearings if required.
Suggest you do a web search to get more information on regupole.
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