I had a couple of vantec stealth fans, which are rated at like 21dB. They were pretty quiet. I don't think you'll get a good idea of how loud a fan is going to be unless you listen to some similar fans in person. Of course, if you add a speed control to it you can easily turn it to whatever speed makes it "quiet enough" for your application. With any amount of road noise, I don't think you'll really be bothered by the noise of a 20-21dB case fan, and probably not hear it at all.