Well, if the noise is coming from the front of the engine and you determined it is coming from a "device on the belt", next step is to determine which device. With the engine running, using a mechanics stethascope, long screw driver or wooden dowel rod, you can listen to each of the accesories individually. Use extra care as it is easy to accidently get wrapped up in the drive belt(s). When you reach the offending accessory you will hear a definite difference in the internal sound - most likely a bearing heading south.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.