I think this is just a normal noise in these cars. Compared to earlier W123 diesels these are virtually silent, but they do make much more noise than the gas counterparts. If this is your first diesel maybe you just need to get used to it. If it is so quiet that passengers can't hear it and the radio will block it out at normal listening levels I would not bee too concerned...if you want to confirm the "normalness" of it just drive another one sometime and compare. Maybe another board member near you has one and you two can see if the same noise is present in their car too.

__________________
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz