You need a passive vent kit.
These are similar to normal vents and wall liners, but have a membrane or special baffle arrangements inside and these prevent draughts and reduce noise.
Try
http://www.airtechenvironmental.co.uk/pages/vapourvent.html and
http://www.passivent.com/downloads/Background_Ventilation.pdf - the models are the "fresh" series on page 42 onwards
The above models have a membrane/filter which is effective in stopping draughts and cutting down on noise. Check with the manufacturer for local suppliers, or they can normally supply direct
The following two are cheaper, but use internal baffles to disrupt the airflow
http://www.airflow-vent.co.uk/draught.htm
http://www.stadium-ventilation.co.uk/vents/downloads/Blackhole_Vario.pdf
Again, check with the manufacturer for suppliers.
None of these are generally common at local merchants, but any local supplier of ventilation products should be able to obtain these or similar ones