I know this is a late response, but I'll say it anyway.

The rear muffler rusting out is very common, especially on vehicles that only do short trips, because the engine doesn't warm up, which then means the exhaust doesn't warm up, which then means condensation doesn't evaporate, meaning moisture just sits in the muffler, and it rusts. (When we moved out of town I was happy because I knew my rear muffler was going to last more than about six months!)

And noise is irrelevant between gas and petrol.