aresh - can you determine which speaker produces the noise?
m0da - you may have an intermittent connection on your iPod connector if it's only one speaker (localized to right front speaker). It may be around Pin 3 on the connection terminals which is an output line to the right channel.
2013 BMW X5 5.0i w/MSport/MPerf/Carbon Blk/Cinn
2011 M3 Cab with DCT / Jet Blk / Blk / CF trim
2008 Audi RS4 6spd Phantom Blk / Grey / CF trim
2007 Z4M 6 spd Interlagos Blue / Blk / CF trim
2006 Z4M 6 spd Blk / Blk / CF trim