btw, one way to check this is to push against the belt tensioner shock while the engine is idling. If the noise stops, you've found the problem...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car