Mazda3 hatchback is fairly quiet ride, compared to Honda Civic Ex -99 that I have. The road noise in the civic is really pronounced. Just add more mats & foam, would be my suggestion. Make sure engine mounting pads are ok. Some unlucky rides comes with the 55 mph vibration.
Mazda3 TSB "Body Vibration at Cruise Speed"
http://www.finishlineperformance.com/mazda...tins_index.html
Wanna add some noise? Try the Hella Twin Supertone Horn Kit. These 5 inch disc type horns will actually fit above the radiator on Mazda3 hatchback 2005 model. Tip: solder a 18 gauge tap wire directly onto the horn relay pin 31, then attach thick 14 gauge speaker wire to the tap wire, to feed these monsters. Simply replacing the relay with a new spare part will undo the mod. Fuse: 25 amps. I left the original horn in place, to add a bit extra timbre. Now 3 horns to play with.
http://www.autobarn.net/twinsuphorki.html