What kind of tires are you running? Does the car pull to either side or pulse? When my brake rotors were really bad I would get a hum everytime I hit them at over 50 mph, even lightly. It sounds like it's probably in the suspension and not the engine, but it's probably not a bad idea to check the front end and make sure nothings loose. Did you loosen anything to install the exhaust and forget to tighten it? Hopefully it will be something dumb like that.