Had the same problem with my 93 w124 300D.. I changed my delivery valves even though they weren't leaking and that cured the "marble rattling" nailing above idle. It also made my idle much smoother! The old orings were flattened and brittle and the copper washers were really discolored. Worth a try??

__________________
1984 190d 2.2 Greasecar Kit. Running Wvo for 25K
1993 300D 2.5T Black on Black. Soon to be Greasy!