I have a 2006 C4 with 49K miles and from a cold start, I began hearing this intermittent grinding noise on top of the regular engine hum during idle. This strange sound would go in and out just like how a locomotive would sound with intermittent whinning noise (which sounds very painful to me by the way). I took off the Air intake assembly to inspect the serpentine belt and the various pulleys, bearings and snubbers. They all seemed fine. I've attached a link to the video so you all can see.

thanks


http://www.youtube.com/watch?v=AOYkz4_LbP0

 

Painful sound during 0:32 - 0:35 in the video