Exhaust gets loud
I have a 2005 Mini Cooper S that when driving after 20
minutes the exhaust gets loud, but if I let the car sit for a
little while it gets quiet again, then gets loud again. What
Maybe you have an exhaust leak. I would check for loose clamps etc.
Given the temperature factor I'd be incline to recommend you look at the catalytic converter(s) first since they are restriction points for hot gasses and often collapse internally.
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