If you do a search on the "diesel clatter" question, you will find some good answers. I 'm pretty sure that the conclusion that your mechanics came to is the same thing that was agreed upon here. The diesel clatter varies from vehicle to vehicle. You cant really compare yours to another. Too many parameters involved, i.e. mileage, maintenance history, how hard the previous owners drove it, diesel fuel used, etc.
My '85 300D seems to be louder when I first start it but after about 15 minutes it seems to quiet down a bit. Some people have remarked that by adding Lubro-moly diesel purge they have reduced the noise substantially. You probably can get it thru partshop.
84 500SEL EURO 87K
85 300D 267K (and still goin' strong)