HAve a 2000 E320w 147.000 miles. after the car warms up and occasionaly at any time with AC on, mostly at a stop or slight acceleration, A little squeal comes from the middle to passenger side of the dash for about a second then goes away, (sounds as if you tore a small hole in a balloon and stretched it out and it squeals) takes spells of doing it frequently, then goes away for no reason. Ive scanned all systems with no fault and checked in car filter for proper operation... any ideas???