Hello Forum! I am interested in getting some test pipes but don't want to have my car getting too much louder. Does anyone know which test pipes would have the least change to the loudness of the car?

All test pipes and HFC's greatly increase the volume of your exhaust, there's no such thing as a "quiet" set. The quietest are the resonated pipes like Berk or Fast Intentions but Berks are crap so stay away from them. Just know that test pipes/HFC's add a significant amount of rasp to an otherwise stock exhaust system so be prepared to deal with that as well. Lots of info in the Exhaust sub-forum that addresses this question.