I have a 2005 GMC safari van i just bought used in april. it only had 12,000 miles . soon after purchase i started hearing a loud banging noise under the front end. this noise was very sporadic. sometimes just at idle or while driving you hear a quick loud bang noise.I've been told by a few people it sounds like a mis-firing catalytic converter. the dealership finally changed the converter after the sixth visit to the repair shop.the noise is still there .It sound like you've just run over something on the road .I'm very frustrated it nver makes this noise at the dealer.HELP what can this be?

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GMC safari 6 cylinder vort