2 good suggestions so far! Thanks, guys. To add additional information, the car is a 1989 300E. The idler pully was replaced 10K miles ago. The belt tensioner shock was noisy, but cushioned the top bolt and it isn't noisy now. In cehcking records, the water pump was replaced 65K miles ago; not 30k.
Can only agree that its the alternator. Hate to spend $200 for a rebuilt only to find out that isn't it. Any way to test if the alternator bearing is bad besides pulling off the serpentine belt and spinning it by hand for noise? Not seeing any play in the pulley with hig engine revs and the belt attached (but wouldn't expect any).
Thanks for the suggestions as always!