I had some odd exhasut related issues appear last weekend, to cut a long story short my car sounded like a 70yr old tractor after hitting a load of standing water on the way to Ultimate BMW. The noise sorted itself after a few minutes. It happened again on the way home after another large area of standing water.
It sounded like the exhaust fell off. It sorted itself after 5 minutes. Very odd.
Theory is the sudden cold water was enough to rapidly shrink the manifold<>centre join gasket and cause the blowing. Then when it heated up, the gap closed up.
And earlier in the week I started to notice some whistling from under my feet on engine braking.
I took it in to my guy, he found the gaskets and flanges were falling apart, corroded.
The studs were grinded and drilled out, flanges cleaned up and new gaskets installed.
It's loads better. Fixed. It's actually quite surprising how much now the exhaust noise is at the rear of the car, rather than escaping from underneath.