You can surround the noisy parts with foam or old clothes or a towel or something, this will help with the vibrations. If you google it there are lots of people who have different ideas on how to silence a fan.

Also, plants don't need any CO2 when the lights are off. If you are only venting to keep a supply of CO2 then you can turn it off during the night. If you have high humidity you might have to leave it on during flowering to stop mould from growing on your buds, but if humidity or temperature is not a problem for you then once the lights go off the fans can go off.