http://www.mass.gov/legis/laws/mgl/90-16.htm
Quote:
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No person shall operate a motor vehicle on any way which motor vehicle is equipped (1) with a muffler from which the baffle plates, screens or other original internal parts have been removed and not replaced; or (2) with an exhaust system which has been modified in a manner which will amplify or increase the noise emitted by the exhaust.
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Basically it says if it's louder than "stock" exhaust, it's illegal.
If you tamper with the cat (KO downpipe), they can fail you unless you have an aftermarket cat
http://www.mass.gov/legis/laws/mgl/90-7o.htm
What you can do is put it back to stock when you go to inspection or
find a place that won't bother.
Where are you in Mass. I know a place that's always easy on me.
If it's not too freaking loud, they don't care.