IIRC from my gasser days, a bad rod bearing will rattle when you let off the throttle (say 3000 rpm), but a bad main bearing will clunk/pound upon throttle up. I've heard that on diesels these phenomena are almost undetectable, and simply let go unannounced.

My money says it's a bad injector, delivery valve, or IP.

I wish I could find $400 MB diesels up here.