Expert: John Kingery - 8/3/2010

Question
Hi, I started to hear a noise like as though someone stick a wooden stick into the fan while it rotates- from under the hood, only when my air conditioning is turned on. WHen i turn off the air condition, the noise goes away. It goes back on when I turn the air conditioner back up. And just recently, my check engine light just came on. So my first question is what is causing this noise that only comes on when air conditioning is turned on, and does this noise have anything to do with my recently check engine light?

Answer
I'm assuming the "wooden stick" sound comes not only when the A/C is on, but also when the heater or the vent is on.  Sounds like you have a twig/seed/something in the rotary fan.  You don't state what model or year your BMW is, but most of these fans are pretty straight forward to get to under the hood.  Clean yours out, if you're sure that's where the noise is coming from.

The check engine is a coincidence, if the prior noise is from the fan.