My guess is that OP has most likely had a US spec 328i Perf muffler installed on his RHD euro spec. US has 2 sets of cats, Euro has one. Putting a US PE on a euro car will mean it will be overly loud and droney due it being designed for a more restricted exhaust system.
He needs to install some resonators in the mids where the secondary cats would be in the US version so as to compensate. Or buy the correct PE for the euro spec cars, that is actually designed for the car.