Fairly new at posting here so I apologize for the long post...Let me first say I have an 04 Cooper S with 108,000 miles on it with an Invidia cat back system. I have had the Invidia exhaust system on for about 3 years and have never had any problems until now. About two days ago I noticed upon start up a sound similar to the chugging of an exhaust leak near the joint between the cat back syetem and the header down pipe. Felt around for a while and cant seem to find or feel the leak. The sound seems to be coming from the cat area inside the piping and goes from a exhaust leak sound at idle to a loud buzz as rpm increases. It seems to go from a buzz to a hiss upon heavy loads with a burble and buzz upon deceleration. Is it possible my cat is dead? or is there something else to look for? any help would be appreciated. No idiot lights or codes thrown and im baffled!!

 

Maybe resonator is blown out? Does anyone know if the Invidia N1 has a resonator or is capable of blowing out? any help.....?

 

Check the exhaust hangers that are located in the middle of the exhaust. Where they attached to the "H" shaped cross member had a little play in on my 05 mcs and it made all kinds of buzzing racket. I had the bracket welded to the hangers and my noise is gone. From your description it's most likely not it, but worth a check.