Exhaust rattle

Hi everyone, i recently noticed a slight rattle coming from somewhere in the exhaust system when the the engine is under a trailing throttle, about 2000 rpm. At my mot last week i asked the garage to check it and he just said, its the middle box thats going, let it develop...i want to get i done whilst i have luxury of time So my question is where should i take it to? ATS or Kwik fit or any other recommendations in the Preston area? Thanks in advance, Elvis