It sounds to me as though engine cylinder
compression is getting past the piston, or escaping the
exhaust system into the crank case. Check for a faulty EGR valve.
If that is fine, then it could be the engine cylinders themselves.
The way to find out if this is the case is to remove one of the glow plugs and spin the engine on the starter, while putting a plastic straw with a paper cup over it into the dip stick hole.
If the cup flies off, or jumps upwards, you removed the glow
plug from a good cylinder. Now remove the glow
plug from the next cylinder and try it again until you get one with a minimal response. That is the one where the cylinder pressure is getting into the sump--with luck as simple a problem as a blown gasket--in which case you will need to find out WHY the gasket blew, or why something got into a cylinder and destroyed a piston, or why the rings are broken--etc etc.
The next thing to check if this all works OK is your turbocharger system. Somehow,
exhaust gas or cylinder pressure is getting into your sump, and if the engine itself is OK then it has to be an EGR valve or if it has one perhaps the turbo system pressure somehow getting into the sump.