I had the same problem with my Laguna when I first bought it, thought the exhaust was comming loose and hitting the car some where. Turn out a few of the fixings on the heatshield had snapped, think it was due to the car being jacked in the wrong place as the fixings looked torn and the rest of the heat sheild was bent out of place. A bit of bending and a few cable ties later and the thing is solid as a rock again. If you can't bend down there I'm sure a helpful friend or relative can get under there to do it for you. Saves paying a mechanic a fortune for a simle fix.