We have an 04 (Sept 03 build ) MCS that is bone stock. The car is mostly town-driven by my wife, whose daily commute is only about 4 miles each way, plus other errands/ trips, etc. Right now the car has almost 12,000 miles on it.

About 2 months ago my wife took the car up to San Francisco (about 45 miles of highway driving) and complained upon her return that the car was much "louder" than she ever remembered it while driving on a freeway. about a week later, we went for a similar drive, and I noticed the same thing----the sound while driving at freeway speeds (65-75 mph) was almost obnoxious.

At first I thought it was the non-runflats (Yoko Es 100 non-runflats) which hadn't been rotated regularily; so after I switched back to the 16" X-lites that came with the car (runflats) the sound was no different. After a test drive with a a MINI service tech (which allowed me to realize it was the engine and not the tires making the noise, the dealer put it up on the rack and came back with a "nothing wrong with it" diagnosis.

I've got 3 other MINIs to compare it to (04 MCS with JCW 15 k miles; 05 MCSa Cabrio, 6K miles, and 05 MC CVT. None of these has the amount of exhaust noise at steady RPM (3,000 and up) that the problem car exhibits. There is a pronounced drone at freeway speeds; and if you rev it at a standstill, you can hear that is much more throaty than any of the other cars, with more resonance and popping---which it never had before. (If it just popped upon deceleration, that wouldn't be a problem!)

There's no discernable hole in the muffler or cat; nor a visible leak. It sounds like the muffler or cat has "failed" but you can't see a thing.

Anyone else experience this issue with a MCS? Thanks

 

Take a close look where any bracket welds are. There are often cracks which are not easy to see. Also, is the O2 sensor tight?

 

My 04' just recently became louder in the exhaust department, i have yet to put it in the air to see whats gone wrong, someone had mentioned before that where the header bolts to the cat back the bolts can become loose. Thats what i was going to look at first. But, I am also curious to see what could be the problem.

 

I have the same issue since a few months ago. Pretty annoying drone when cruising at highway speed, and very loud when reving in the garage.

Gonna ditch the run-flats and see if it helps, otherwise I'll probably get a muffler shop to check it out.