Originally Posted by JCL

You will always have additional noise with the diesel, and there isn't a way to get rid of it (except, as others have said, buy a gasoline engine instead). The noise is much less than with older generations of diesels due to the multiple injection events and better managed combustion process.

I would be interested, however, in whether it has changed while you have owned the vehicle. If not, then there is not much to do about it. You can tolerate it (thinking always about the fuel economy), or you can celebrate it (imagining that you are driving a heavy truck). Both approaches have been described here by different posters, and posters who are fans of the diesel tend to fall into one of those two camps.

If it has changed, I would look into the fuel quality I was buying (cetane number) and see if different fuels from different suppliers had any impact on the noise upon acceleration. The cetaine rating describes how the diesel ignites, and low cetane numbers will increase engine noise. Not all diesel fuel is the same. Cetane isn't the same as octane rating, but it is somewhat similar in concept.

I don't think there is any significant difference between the 335d and X5 35d in North America since they both use the M57 engine. If you are comparing a new model (overseas) of the 335d, then you could be comparing the newer N57 engine to the older M57 engine, and I would expect them to have different sound levels.