Regarding feedback (engine whine) and the Belkin connector:
VERY common problem. The Belkin connector uses the same ground line for the power and the audio. This creates a large ground-loop (basically, a huge antenna inside the car) that picks up all the AC noise in the electrical system.
There's a very easy fix for this: purchase a connector cable that incorporates a "ground isolator" (sometimes called an "audio isolator"). Radio Shack sells this cable for about $15, looks like a normal RCA cable with a 1" x 3" cylinder in the middle (containing the isolation transformers). Works perfectly with the Belkin connector. This is how I attach my Belkin/iPod to the aux-in connector on our 335i. Note that the Radio Shack cable ends with RCA plugs. The kit includes one RCA to 3.5mm miniplug adapter; you'll need to purchase another to get both ends to miniplug configuration.
Side note: I'm *amazed* no one has offered what I consider the ideal audio solution for iPod-to-aux input connection. Incorporate the functionality of the Belkin connector (charge + line out on the plug; allows only a single cable to the bottom of you iPod) and include a ground-loop isolator as part of the package, built into the car-charger portion. Frankly, I'd think you could sell thousands of these things with ease...