Tsuyoi is right and i would have to agree with him...
Lines or no lines, there is no difference in terms of quietness..
hAVING THE 335i, i can tell you that the engine sound with the hood open on the 6 cyl DOES NOT sound as sweet as on previous 6cyl 3 series.
Both current F30 N20 and N55 sound terrible claking and rattling due to the turbo affixed.
eXHAUSt-wise, the 328i would be less melodious than the 335i.
The 335i is bordeline hooligan with its exhaust, (although not as much as the G35/37)
If you want the quietest car possible, you should consider Lexus or the 7 series...
3 series is not well insulated at all compared to the competition...