So I’m not alone!
Did they accept it as a fault ?
The most frustrating thing is having paid toward fitting and I imagine return delivery won’t be cheap
Just a thought for the OP, the exhaust isn’t touching the underside of the car somewhere and causing it to resonate? The new fiment might have caused issues further back up towards the engine?
I have the IL motorsports cat back stainless steel exhaust for my NA 1.8 Euro’s and I absolutely love it. Is quiet with the roof up and cruising at a steady speed but lovely and raspy with the roof down on accelerating. No drone at motorway speeds and did 2,500 mile trip to Italy and back with the roof down for 95 % of the time and did find it all tiring and more to the point my wife didn’t make any negative comment at any stage. Clearly these opinions are very personal but I am generally pretty sensitive to irritations such as noise and rattles but re the exhaust I cannot be happier with my choice.
Cheers
Matt
Just wanted to follow this up with an outcome; positively 
Yes - the cobalt is louder than many reviews make out and people who state it is barely louder than stock are straight up lying to you haha…
however - the exhaust has bed itself in very well; and is far less miserable solid tone and more occasional burbles which have finally given it character and a decent note: big difference overall since fitting and sounds lovely .
I said earlier in the thread give it time to bed in. If I remember I had doubts whether it was up to what I’d read about sound wise but it came good after a few hundred miles.
No kidding the previous back box I had fitted rattled the garage door and the glassware in our wall cabinet in the lounge (it’s on the garage wall side of the house) when I pulled the car out of the garage. Also my OH complained about the boom too. No such complaints about the Cobalt, she loves it.
Phil I have a 2016 Recaro ND and am thinking of the Cobalt back box from MOss at £245… how is it going for you and did you fit it yourself? Lots of differing thoughts on this site! Appreciate your comments back!
For me it was perfect …sporty start up it gets better with use …Nice sound on acceleration but quiet cruising and no bomming…very easy to fit no more that 30mins just spray you rubber hangers with wd prize a small screwdriver in the rubber to lift it of the bracket make sure you get the spray all around the hanger… bet getting them off causes you the most assell but mine was only 12 months old when I did it…so plenty os spray every were
My cobalt was a little boomy just replaced the inner copper o ring (mx5 parts) and a new gasket (cobalt service parts) and its fine wouldnt of fitted this myself but it came with my nc when purchased