On my 02 neon I put on aftermarket headers and downpipe and replaced the cat with a flex pipe. the leak is where the new down/flex pipe fits together with the old piping. I noticed my gas mileage is greatly reduced on in town driving. on the other hand my highway mpg seems to be normal or at least close to normal. Thanks alot in advance.

 

It can, yes, especially on a car with O2 sensors.

 

If the leak is before the sensor or the cat yes, if not then it's just probably loud.