Welds on the catalytic converter heat shield can corrode causing a rattle during acceleration and possibly causing Check Engine Light illumination. The common repair for this issue is replacement of the catalytic damaged catalytic converter.
Car Problem Reports
Nissan Altima Rattle/CEL Due to Cracked Welds on Catalytic Converter Heat Shield
Nissan Altima Problem
Exhaust & Emissions Affected: 3.5L V6
Average Mileage: 56,012 mi (30,000 mi - 89,000 mi)
Visitor, 2007 Nissan Altima, 40,100 mi
The heat shield welds came loose on catalytic converter which was causing a loud noise. Had it fixed for under $100.
Visitor, 2008 Nissan Altima, 63,000 mi
Had to replace the entire catalytic converter for $800 including parts and labor.
Visitor, 2007 Nissan Altima, 66,000 mi
Have a rattle upon first starting my Altima. Goes away after it warms up a bit. Haven't fixed it yet but looking for a solution
Visitor, 2007 Nissan Altima, 36,000 mi
Visitor, 2006 Nissan Altima, 57,000 mi
Rattling noise at low speeds, usually early in the morning. Diagnosed as a heat shield problem and fixed for pretty cheap.
Visitor, 2008 Nissan Altima, 30,000 mi
Rattle occurs on engine start. Typically occurs on a cold engine.