[Broder]

Same problem here. 2006/2007 330i EU-Spec with 66.000km (41k miles). Though some of my symptoms seem to be different. Here´s what happens:

When I have the A/C turned off (engine idling @ 600rpm on stops), I usually get a annoying vibration that you can feel mainly through the seats. It happens always, independent of the engine being hot or cold. Solution? Turn the A/C on, idle RPM increases and the vibration ceases.

Now the odd thing, when I turn the A/C on I´m sometimes able to hear a metallic noise coming from the engine bay and you can feel a little vibration from the interior of the car, although it´s a DIFFERENT vibration from the one that happens when you have the A/C turned off, the metallic vibration that happens with A/C on is much less pronounced and definitely better to live with then when the A/C is off (that´s why I always run with the A/C on, even when it´s not necessary). This metallic noise/vibration seems to be worse on hot days (or when the engine is hot) and less pronounced on cold days, so sometimes you won´t feel it at all while at other times it will bother me quite a lot. However, this metallic noise/vibration is also discrete, the engine itself tends to be noisier then that sound (that´s why, even from the outside, people usually can´t hear it), people that ride with me usually can´t hear it or feel it even when it´s there, I can hear it because I drive the car on a daily basis and have to live with it, so I´m naturally more sensible to my car´s noises and rattles then other people, this also means it´s hard to diagnose the problem in a BMW dealership.

Once, arriving home in a silent night, the sound was actually pronounced enough that I was able to hear it out of my car. I found it the easiest way to listen to it is to crouch and hear it placing my ear next to one of the front wheels (sounds louder on the driver´s side), it´s easier then trying to listen it from top of the engine bay (since engine noise is very loud from the top part), this metallic sound is definitely caused by two elements that are "touching" each other when they shouldn´t, although I can´t pinpoint exactly where it comes from. My first natural hint was that I probably had one or more engine mounts busted, and it was probably causing the engine´s downpipe or something alike to be in contact with something else, causing that metallic noise/vibration.

Well, the technician at BMW told me my engine mounts are fine, there´s nothing wrong. And pinpointing the metallic noise/vibration seems rather hard at BMW, specially since, for some reason, my car is always very well behaved at the dealership. Turning the A/C off seems to eliminate the metallic noise/vibration, but starts with another kind of vibration that´s apparently caused by N52´s excessively low idle RPM, and I thought "perhaps both of these vibrations are related, and fixing the 600rpm no-A/C vibration will probably also fix the metallic A/C vibration together", so I turned off the A/C and showed my car´s vibration to BMW´s technician. He agreed that the vibration seemed "wrong" and they checked the car for problems.

The next day they called me telling me the problem was the engine alternator. I thought "wtf, the alternator?". I said "there must be something wrong, the alternator can´t be doing this, it makes no sense", the technician then told me "yes, it´s weird I know, but your alternator seems to have internal loosen parts, that causes the excessive vibration when the A/C is off, once you turn the A/C on, the vibration ceases since the load caused by the A/C causes the alternator to work tighter".

Ok, since my car´s on extended warranty they changed the alternator for me. That was some weeks ago, did it solve? Not really, though some things did get better. For instance, whenever my car was idling with A/C on, sometimes the RPM would have a sudden drop (A/C compressor being activated perhaps?) for a few seconds, then go back to normal range. Since I´ve changed the alternator, I have noticed no drops or oscillations whatsoever during idle RPM. Idling with no A/C seems to be better also, but it´s still uncomfortable, and with A/C on I can still feel the metallic noise/vibration... gonna have to take the car back to the dealership when I have the time...