I wonder what country you're in? The domain name doesn't tell! We here in
the US are jealous of your X4!
Anyway, I've always thought my 1989 Mi16 could be quieter than it was but
it's no where near as bad as you're finding.
I think one source is the tubular exhaust header. They damp should a lot
less than cast iron. Consider various firewall insulation schemes or even
some of the racer's header insulation wrap. A stethoscope search by a
passenger might lead to soemthing.
-Jim