New belt tensioner pulley. Common cause of noise at front of engine when bearing starts to go. Should be replaced at 120K as a preventative measure.
601 602 603 Diesel Belt Tensioner Pulley
$26.95
SKU:
DSL-COM60BP 
Product Details
Tensioner shock should be replaced at the same time. (look below to order). This pulley fits all the all-aluminum 4 (601), 5 (602) and 6 (603) diesel engines from 1984 to 1993.
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190D 300D 300SD 300SDL 350SD Belt Tensioner Shock
This is the number one cause of a sharp engine rattling noise at idle on the early 602, 603 and 604 engines. It is not that the shock goes bad, but the bushings car wear out prematurely due to harsh vibration and the ends of the shock will make a terrible noise inside the loose bushing.