Well, I've never heard of anyone getting done for loud exhausts, sure there are legal limits, but the cops do not carry the testing equipment to prove or dissprove the noise levels, there are also so many other mitigating factors, background noise, large buildings will reverbirate noise and increase level, (tunnel effect). You'd have to be really unlucky to get a fine issued for a loud exhaust. There would usually be other reasons behind the officers actions, such as "you were driving like your arse was on fire". Even if one did give you a ticket for something so trivial, I would argue it in court, there is no way you will lose.