BMW Repair: E46 heater noise, ventilation system, heater core

Question
Evan,
I drive a 2000 328i (E46).  During much of the year, the ventilation system works fine.  However, I've noticed that once the external temperature drops below freezing, the heater makes a high-pitched chirping noise when I start the car, kind of similar in sound to a hyperactive cricket.  As the car warms up (5-10 minutes), the sound lessens, but doesn't completely go away.  What are your thoughts on the origin of the sound and whether it's something I can repair myself?  p.s. I used your instructions for replacing a window regulator unit on this car and they worked great!

Answer
Jim, Please make sure that your cooling system is full and eliminate any leaks. When there is air in the heater core it will make a similar noise to what you are decribing. As the motor warms the coolant it expands and pushes the air out lessening the noise. Congrats on your successful window regulator R&R.
-Evan