When I installed the Carbonio intake from Randy in my '99 E36 328iC (M52) it was a LOT louder than the stock airbox.
When I installed another Carbonio intake from him in my '04 E46 330ci (M54) it was only a bit louder than the stock airbox, with the biggest difference and really only very noticable being above 5,800rpm.
I guess it depends on the engine, really. That and I could hear a lot more noise with a convertible with the top down than the much more refined E46 with an actual roof, haha.
Past: Fern Green E36 328i Convertible, Imola on Alcantara E46 330ci M-Sport, Dunkelgruen E36 328i, Le Mans Sunset 350Z
Current: E36 M3 Convertible - California Import, Estoril Blue, Dove Grey, Staggered Style 32s, Muffler Delete, Koni Coilovers