Is it a constant odd noise, or is it an odd noise at certain RPMs? I know my heat sheild has rusted through (live on the coast), and it makes an odd noise at certain RPMs. But if it is just a plain louder sounding engine noise than before, then it could be a hole in the pipe, or muffler... could be a lot of things. If you don't know for sure, just take it to a muffler shop and put it up on the lift. They should be able to tell you without a whole lot of replacing.