Weird exhaust sound

[PetitPimousse]

Hey forum,

Sorry but I am no expert. I purchased my 08 m3 used, and it came with a Remus exhaust (not sure which version of it).
When I accelerate and let go of the gas I hear some form of backfire sound, which can be pretty loud. Is that something to worry about?

Also, when i purchased the car, the BMW dealership told me that the exhaust would not void the warranty. Is that correct? (the warranty is quite a distance away so I can't really take my car to them if the need arises).

Thanks for the help!