2003 Mini making noises like a deep fryer while idling

My 2003 Mini One (1600cc engine) started making noises recently. Once warmed-up and idling, it begins making increasingly loud noises from directly under the car, which sound like a deep fryer. It stops when going above 20kph. I thought I had a loose heat shield or something else wrong with the exhaust system. When I took it in to the ONLY mini dealer in the country (I live in Singapore), they of course diagnosed it as a fix that required replacement of the US$2000 catalytic converter. Does anyone have experience with the symptoms? Does it seem like a catalytic c. replacement makes sense? The car has only 63000 kms on it (appr 40K miles).

Whoops- I forgot to mention, when the dealer hoisted the car up on the lift, I did have an opportunity to check the heat shields - they all seemed solid.