I have a metallic crackling noise coming from what sounds like the header area, it only occurs at wide open throttle. Best way to describe it is, rocks rattling in a pipe. The car has tubular headers and a cat back. Had the car up in the air and have not noticed anything loose. All pulleys on the engine are quiet, only time I can replicate the noise is on full throttle. The car has a ATI super damper, I heard of the cars having issues with the stock pulley. The belt tensioner has been replaced in the last 30k miles as well.

 

Either a heat shield vibrating, your cat has started to disintegrate or your resonator may have loose baffle/s. If it is your cat and it clogs, it will shut you down.

 

To monitor the car....if you keep an eye on boost psi...
You will see you boost levels start to climb as the cat fails and becomes plugged or melts.

 

I have a header in the car with a 200 cell cat. Seems to be like it also was installed in the last 20k miles. The though has crossed my mind, took a mallet to it and it has no loose honeycomb. Its distinctly up in the firewall area .

 

Many/most aftermarket cats tend to fail in the gen1 MINI.
Most of the metal ones melt...they are made too cheaply...and the ceramic ones are cracked and broken by the harmonics of the motor...