Sounds like oil, as in engine oil, but these are usually long lasting, rattly as f**k but long lasting, it can also be caused by a slight amount of petrol in the diesel, if you are talking very light blueish smoke, that gets worse under acceleration ?
I would in any even drain off the water from the filter head and check the diaphragm in the filter head has not gone porous, very very common.
tabetha