back in japan i know some people actually ran a motor cycly muffler on the end of the tube to quiet it down. i dont know how much it would work or if our cars have that much room. they even fabbed their own resonator on one but it looked ghetto.

 

Just run it out the back with the exhaust. Most of the sound will be directed away from you. You can use a motorcycle muffler, but don't restrict the flow. Have it merge in into the exhaust pipe right before the muffler. It will be quiter and not provide any backflow. Just make sure the pipe doesn't come in at a sharp angle.