this is a crap shoot but...go to the cops and ask them what the local laws are. if possible, get the car tested by them. if it's too loud you just shot yourself in the foot, BUT, if you get pulled over they'll know that you got it tested. this means you're trying to do the right thing, they don't expect you to fix things instantly, and if someone complains the cops are more likely to ignore it. obviously you will have to get it fixed though.
but if it's legal, you can tell everyone to go screw.
but if it's legal, you can tell everyone to go screw.