smoke around 40mph

[00white73]

Hello all,
I am new to this forum but not to the forum world. I have an issue with my 2000 7.3 which is stock with no mods and 67xxx miles on the clock. For a little while now it blows white/ blueish smoke intermitenly. I have noticed it will do it most often when I am decellerating and at about 40 mph it will blow a few puffs of white/ blueish smoke till about 30 mph and then will go away. I have no power problems and will start everytime. I am at a loss at this point. I dont know if this is normal or an issue. I have changed the oil, fuel filter and put in a fuel addative for XDP. If anyone can shed some light it would be appreciated.

Thanks.