I may be way off base here, but I have read elsewhere that noisy lifters on other models, especially older diesels, can be substantially reduced by a switch to synthetic oil, like Mobil 1. IF your noise is in fact lifter-related as suggested above, might be worth a try.
I switched over at 80,000 miles on my 400E. . .
[Edited by 400E on 06-16-2001 at 08:44 PM]