So the weather is warm once again and everyone will have rolled down their windows for the first time since Halloween and lo and behold there will be many a new E46 driver that will hear a loud high pitch cold engine whistle/squeal at acceleration and then they will come here and start a thread about it without searching first and thus add to the hundreds of already established threads on engine whistle.
All you need to do is go down to your BMW Dealership and buy a new Air Intake Rubber Boot Tube Elbow (BMW P# 13-54-1-438-759) for about $12-18. Next buy a small thin 6mm wrench or ratchet and follow this simple pictorial DIY: BMW330ci.net - Tube Elbow Replacement (Do-It-Yourself)
Chances are if your car has over 50,000 miles AND is approaching 10 years on the road then your Tube Elbow is already ripped, has a hole in it or in the process of ripping.
Here is what mine looked like and my car only has 55,000 miles on the odometer.