It' prob nothing minor.
Have you noticed any white, grey, or black smoke out the exhaust? Or a significant loss of power (like if the computer has shut down a cylinder or two)?
If not, it may be similar to the following in other's vehicles.
http://www.powerstroke.org/forum/99-03-7-3l-general-discussion/26311-diesel-fuel-coolant.html
(Message edited by roomservice on March 08, 2009)