Exhaust System---replacement

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I know this is for aftermarket discussions, but I'm not looking to upgrade just yet...I'm looking for a long standing temporary solution for my 98 318ti. Currently I can smell exhaust (so there's a leak somewhere...MD roads are crap so EVERYTHING RUSTS.) I think the heat shielding (baffle) is what's rattling, because I hear a rattle when I accelerate and the check engine light is on for the second time in 2 weeks I was told that the o2 sensor downstream was replaced, but I don't know if I believe it, as 10 miles later the Check Engine light came on a second time... So I think I need to replace the other o2 sensor.