check your wastegate
Mine did it the other day!
Take the boost pipe off and get a high pressure line into it and give a good blasting!
After spray it with wd40
Thats what I did and it cleared up !!

Diesels do ned a good clear out once in a while, when its mot time is best, give it a good thrashing on way and see all the crap come out, then be amazed at how low the emissions will be when it gets tested. My last 4x4 was a toyota and it had 1/5 of the emissions of a 4 year old 4x4 that was tested before mine, mine was a 1990.