Not Stable Sound From Exhaust/ Bad Smell in the morning

[RMoove]

My car is a BMW 530 2001 (E39)

The other day I noticed that the sound of my exhaust at the back is choppy in idle, the sound is not stable, although the engine is stable not shaking or vibrating and the RPM is at a healthy 800-900. What could cause the exhaust to act like that? Also there is a bad smell in the morning when the engine is still warming up.

Appreciate any advise as my local mechanic is clueless.

[Pelican Parts]

Are you able to post a video of this? Does the car drive fine? Try to get the codes read, it'll give you an indicator if and where the issue is coming from.