Driving down the freeway and engine developed a knocking sound.

The owner has only witnessed it once himself. The vehicle came in on a tow truck and drove fine off the truck and on a test drive. Found that the DPF was at 16% so a forced regeneration was performed. At the point, the regen said successful the engine started knocking and carrying on. Seems to happen 9 times out of 10. The engine creates a bad diesel knock, runs rough, stalls and white smoke comes out of the intake from the turbo.